Why be this way?

I'm not Beaver, but I think they're understandably frustrated at consistently being berated for trying to do good by adherents to a prevailing belief system whose widespread harm goes almost completely unchallenged by society. You can see that even this post in a vegan community which is nothing but a polite ask followed by robust scientific sources is at a 75% upvote ratio because veganism is so irrationally hated by society at large that overwhelming scientific evidence for its healthfulness is a target of disapproval.

is the vegan day-to-day so bad

Pretty nice overall. Food's cheaper, you tend to feel like you have more energy, you often end up discovering interesting and really great new foods that you wouldn't have with a diet revolving around animal products, and your GI tract works like clockwork. Super bizarre, that last one, but I can't not mention it.

made up pseudo-slur

All slurs are made up, but this isn't one inherently; "carnism" is just a way to describe the ideology "in which people support the use and consumption of animal products, especially meat. Carnism is presented as a dominant belief system supported by a variety of defense mechanisms and mostly unchallenged assumptions". (Quoted from Wikipedia because they summarize it excellently.) It's a way to challenge that the idea that "veganism" and "vegetarianism" are ideologies while carnism is just "normal". It can be used as a slur, but that's every word for an ideology ever.

I could be convinced to change my diet, but

You should know that vegans get this all the time. Because vegans care about animals and the environment (the latter of which of course helps the animals too), this comes off as "you better police your tone or the animal killing will continue, and you wouldn't want that, now would you?" This could work, because ultimately all vegans want is to see as few animals exploited as possible, but it can ring hollow for reasons in the next paragraph.

it's not even possible to engage vegans where they are

People very often engage extremely politely outside of vegan subs in relevant areas (e.g. posts to news articles about animal welfare) and are downvoted to hell for it, their voices drowned out by ridiculous, bad-faith pseudo-arguments which are easily debunked and regurgitated so often that there's a running joke about vegan bingo. It's frustrating to vegans when carnists have an absolute right to make fun of vegans as malnourished zealots whose way of living is often compared to a fate worse than death, is tantamount to child abuse if imposed on one's children, and is so tenuously held that they just need to "try a cheeseburger" and wake up out of the Matrix. But if they so much as speak out of turn (like what Beaver said is pretty tame in the grand scheme of things, right?) after being berated constantly for just existing, let alone politely broaching the subject where it's relevant, then this is why nobody likes vegans and we were right to make fun of you this whole time and just for that, I'm going to go kill a chicken and vegans are religious extremists.

I'm never going to subscribe to the vegan religion

Vegans tend to distinguish a plant-based diet from being vegan, as vegans stick to it as a way of promoting animal welfare, but more importantly, there are products outside of their diet such as leather that they don't consume either. So you're interested in a PBD, which is still a huge reduction in harm. People who haven't tried one often treat a PBD as a huge commitment, but as a hobbyist cook with not a lot to spend on ingredients, it's one of my favorite parts of being vegan. I have a really hard time making and sticking to lifestyle changes, but I consider it one of the easiest things I've ever done because I feel better, and I enjoy the food I cook and eat more than before. You might be interested in /c/homecooks on vegantheoryclub.org or /r/veganfoodporn on Reddit.

I am interested in the principles and the "how to".

For a while, I landed on the idea that I would be pescetarian and stayed there for a while. From a health perspective, a lot of the benefits of a plant-based diet also come with the so-called Mediterranean diet, and so that's what I wanted. I weaned off of fish largely for environmental reasons at the time, although I also decided that if I wouldn't want to kill a fish with my bare hands, I wouldn't want to pay someone else to do that either. And lastly, I stopped consuming animal products altogether after watching the Dominion documentary. I was never vegan because I was of the opinion that it only slightly reduced the amount of harm from vegetarianism and that vegans were bitter idealogues trying to enforce purism. After the documentary, however, I realized that instead of being 90% of the way there, I was maybe more like 50% if I'm being generous. And so I started cutting out animal products like dairy.

As for "how to maintain that", I think I had a good experience sticking to it because I was able to focus on eliminating specific foods and how to replace them rather than just being thrown into figuring out how to replace large components of my diet. While it is entirely possible to get on and stick to a plant-based diet cold turkey when doing it for ethical reasons, if it's strictly for dietary ones, then I suggest just cutting out red meat first. This is what I did, and I think it's a good starting point both if you're concerned about ethics and the environment and if you don't care about either of those and just want to be healthier.

The key to a PBD is nonperishable staples like legumes, nuts, seeds, and grains. Those can be paired with fruits and vegetables. For plant-based milk, I highly recommend Almond Breeze unsweetened (not vanilla), as I think this is about as close to milk as you can get without drinking the sweetened kind (which destroys a huge health benefit of switching to plant milk) or drinking pea milk (which is the best but is expensive). But if almond milk isn't your thing, the amount of variety for plant milk is so enormous that you'll probably land on something you like if you keep trying. I think it's paramount to have a wide variety of dry spices bought in bulk, because these along with sauces take what can be boring foods and turn them into something incredible. This isn't so much a concern for a health-centric PBD, but getting mired down in plant-based alternatives to animal-based ones (I think with the exceptions of milk and yogurt) is the easiest way to make it as hard as possible, because it's really not sustainable if you have any sort of a budget. Lastly, knowledge of what non-homemade foods are plant-based just comes with time, but if you're doing it for health reasons rather than ethical ones, the fact that milk powder rears its head in places it has no business being would be much less of a concern.

This is a great hook post

Much appreciated. I think the stigma of vegans as people physically wasting away for their beliefs ought to be addressed first because it's really just incorrect, and while there's a kernel of truth because nutritional deficiencies can happen if you're extremely stupid about it (fruitarians come to mind), I think a huge part of it comes both from industry propaganda and from something people can hide behind so they don't have to confront the ethics of animal agriculture ("well it's that or being healthy, so..."). It's possible to go plant-based and have nutritional deficiencies, but it's also possible to have a terrible, nutritionally deficient omnivorous diet. If done right (which shouldn't be a lot if you don't have some sort of medical issue; I supplement B12 via nutritional yeast and am otherwise pretty loose with my diet, and my labs are always great, even much better than before), you can easily be healthier than on an omnivorous diet in huge part thanks to the reduction in saturated fat and the increase in unsaturated fats and fiber.

The technician did a great job replying to most of what you said, but can I just add one more thing which helped me see veganism from a different perspective, in response to you calling veganism a religion (I know you changed your view already and probably didn't mean it literally but I just wanted to address it anyway):

Apart from the obvious that veganism (which I prefer to consider "the animal rights stance") is an ethical position/social justice movement more alike to something like feminism or pro-LGBT rights; and doesn't have any spiritual beliefs attached to it and is based purely in ethics/compassion, philosophy/logic/reason, as well as science/evidence (for the related environmental and health components), meaning it doesn't really cover any of the hallmarks of a religion unless we consider other, human rights-based justice movements religions too...

I almost see veganism as being the opposite of a religion, not just because it frequently rejects religion as being an excuse or justification for violating individuals' rights (though it is compatible with religion and there are arguments for veganism from religious perspectives like there are for other rights-based positions, e.g. the Quakers were actually pivotal in abolishing slavery in the US, and progressive churches make a case for homosexuality being accepted and for it to be sinful to victimize people on the basis of their sexuality, etc)...

But because veganism confers the ABSENCE of dogma, not the presence of it: that dogma being the normalized, ingrained societal/cultural belief system that accepts and assumes not just the superiority of humans and lowered importance of non-humans (human supremacy/anthropocentrism), and the differential perceived-value & treatment of certain species of sentient beings based on factors like their utility to humans or their endearment to us (speciesism), but also accepts & even promotes (and largely opposes the rejection of) carnism, or the systemic exploitation of & cruelty toward non-human animals for various purposes, which utilizes the "four Ns" of carnist conditioning as a validation mechanism; that to exploit animals for their flesh, secretions, skin, fur, etc. is "Nice, Normal, Natural, and Necessary"... which are views based not on science but on a willingness to believe in things without evidence or reason, often that suit one's pre-existing narrative and are convenient to enable them to maintain control over less powerful members of society, or vulnerable/innocent individuals/victims and continue acting according to the status quo (which is unfortunately how religion has often been used, though not inherently, and sometimes in the opposite way).

In this regard, you could argue that veganism is to carnism, what atheism/agnosticism is to religion/theism. I hope this makes sense.

It’s the animals who are the victims not you for being called Carnist. You have same energy as boys on twitter complaining about being called cis.

Also veganism is a philosophy that’s been around since 1944 and carnism was coined by the amazing physiologist Melanie Joy.