We feel conscious and have an internal experience

It does not make sense to add the qualifier "internal" unless it is being contrasted with "external." It makes no sense to say "I'm inside this house" unless you're contrasting it with "as opposed to outside the house." Speaking of "internal experience" is a bit odd in my view because it implies there is such thing as an "external experience". What would that even be?

What about the p-zombie, the human person who just doesn’t have an internal experience and just had a set of rules, but acts like every other human?

The p-zombie argument doesn't make sense as you can only conceive of things that are remixes of what you've seen before. I have never seen a pink elephant but I've seen pink things and I've seen elephants so I can remix them in my mind and imagine it. But if you ask me to imagine an elephant a color I've never seen before? I just can't do it, I wouldn't even know what that means. Indeed, a person blind since birth cannot "see" at all, not in their imagination, not even in their dreams.

The p-zombie argument asks us to conceive of two people that are not observably different in every way yet still different because one is lacking some property that the other has. But if you're claiming you can conceive of this, I just don't believe you. You're probably playing some mental tricks on yourself to make you think you can conceive of it but you cannot. If there is nothing observably different about them then there is nothing conceivably different about them either.

What about a cat, who apparently has a less complex internal experience, but seems to act like we’d expect if it has something like that? What about a tick, or a louse? What about a water bear? A tree? A paramecium? A bacteria? A computer program?

This is what Thomas Nagel and David Chalmers ask and then settles on "mammals only" because they have an unjustified mammalian bias. Like I said, there is no "internal" experience, there is just experience. Nagel and Chalmers both rely on an unjustified premise that "point-of-view" is unique to mammalian brains because supposedly objective reality is point-of-view independent and since experience clearly has an aspect of point-of-view then that means experience too must be a product purely of mammalian brains, and then demands the "physicalists" prove how non-experiential reality gives rise to the experiential realm.

But the entire premise is arbitrary and wrong. Objective reality is not point-of-view independent. In general relativity, reality literally change depending on your point-of-view. Time passes a bit faster for people standing up than people sitting down, lengths of rulers can change between observers, velocity of objects can change as well. Relational quantum mechanics goes even further and shows that all variable properties of particles depend upon point-of-view.

The idea that objective reality is point-of-view independent is just entirely false. It is point-of-view dependent all the way down. Experience is just objective reality as it actually exists independent of the observer but dependent upon the point-of-view in which they occupy. It has nothing to do with mammalian brains, "consciousness," or subjectivity. If reality is point-of-view dependent all the way down, then it is not even possible to conceive of an intelligent being that would occupy a unique point-of-view, because everything occupies their own unique point-of-view, even a rock. It's not a byproduct of the "conscious mind" but just a property of objective reality: experience is objective reality independent of the observer, but dependent upon the context of that experience.

There’s a continuum one could construct that includes all those things and ranks them by how similar their behaviors are to ours, and calls the things close to us conscious and the things farther away not, but the line is ever going to be fuzzy. There’s no categorical difference that separates one end of the spectrum from the other, it’s just about picking where to put the line.

When you go down this continuum what gradually disappears is cognition, that is to say, the ability to think about, reflect upon, be self-aware of, one's point-of-view. The point-of-viewness of reality, or more simply the contextual nature of reality, does not disappear at any point. Only the ability to talk about it disappears. A rock cannot tell you anything about what it's like to be a rock from its context, it has no ability to reflect upon the point-of-view it occupies.

Although you're right there is no hard-and-fast line for cognition, but that's true of anything in nature. There's no hard-and-fast line for anything. Take a cat for example, where does the cat begin and end, both in space in time? Create a rigorous definition of its borders. You won't be able to do it. All our conceptions are human creations and therefore a bit fuzzy. Reality is infinitely complex and we cannot deal with the infinite complexity all at once so we break it up into chunks that are easier to work with: cats, dogs, trees, red, blue, hydrogen, helium, etc. But you always find when you look at these things a little more closely that their nature as discrete "things" becomes rather fuzzy and disappears.