Never even heard of it

Why don't you set up a Matrix server with the bridges you use then you will get exactly that, one app to chat with every other chatting app.

I've gotten access to it, tried it for all of 30 seconds, and immediately deleted after. For one, there is a difference between full app integration and "you can connect it through beeper", which essentially removes any feature except for basic messaging. But the most egregious issue is that if you decide to try to use their iMessage service, they just host it on some random Apple hardware they have, guaranteeing absolutely 0 security

I waited for ~2 years to get onboard and just having the ability to use iMessage on my Pixel 6 has made it worthwile. Plus the tight integration with other services and being able to use it to sync SMS between devices is like Apple level. It's probably not the best service for certain aspects (e.g. privacy, they say it's E2E but who knows what their stack is), but the secret sauce that is the app just ties it all together so well. Plus, they have support built right into that app via a private Matrix Room and a REAL PERSON will respond to almost any query within a couple minutes. I accidentally sent a DM request to the CEO and he replied in like 3 minutes. If you want to skip the line I can give you my invite code so you can try it out.

people are posting invite codes all the time, I found one after looking for about 10 minutes.

don't really like it much though, and thankfully none of my friends use imessage.

Any of these jack-of-all-trades apps are never very good. There's a reason why dedicated apps to a single messaging system requires so much work, there's a lot to go into it.

I accidentally signed up with two different emails and got in within a week on both. You sure you signed up with a functioning email address?

I set it up all excitedly on my tablet and proceeded to never use it. And just wait til you see how bad it is on chromeOS. The fastest way to launch it is through command line.