Having an internet-connected TV running Android is great. With this, I can download and install SmartTube, and then use it to watch YouTube without ads or sponsor segments. I can also install Jellyfin, so I can watch various media served by my PC.

You're better off using an Nvidia Shield for that. You can swap that or the TV out independently as tech changes. You can also plug drives into it and play whatever media you want.

Except that my TV already has a computer running Android inside, so why would I want to spend a bunch more money on an additional device that just does the same thing?

If I could have saved a little money by getting a 65" monitor, this advice would make a lot of sense. But that's just not an option these days. You're getting a built-in computer whether you want it or not.

As for plugging in drives, it looks like you can only do that with either microSD or USB. Either way, doing that for every item I want to watch is very inconvenient compared to just using Jellyfin, which lets me browse everything on my PC.

Not at all. I have an SSD plugged into my Shield, to which I can copy whatever I want over my network. People also like to install torrent-based clients for illicit streaming services, although I don't consider that worthwhile.

Your observation about every TV forcing built-in junk on you is legit; but I use a projector, which doesn't have any of that bullshit. Every source I have goes through my receiver, which is another reason to use a media player that's not your TV.

The whole thing is a sad commentary on the state of A/V and people's laziness today. They're buying giant TVs but settling for the shitty audio from its built-in speakers... or fooling themselves with a sound bar.

I have a sound bar. I'm not fooling myself. It's much better than the built-in speakers (though my soundbar has a small subwoofer, so that helps a lot). Is it equivalent to a real A/V receiver/amplifier with large speakers? Definitely not, but that setup costs a LOT more money, plus I'm limited in how much I can turn up the volume anyway because of my neighbors.

The projector thing is a no-go for many people. They cost a lot of money, the bulb inside has limited life and is very expensive (and makes a lot of heat too), but worst of all, they just aren't bright enough for many viewing environments. I would never be able to see it in my apartment, except late at night.