I'm using NewPipe [1] but it's TV support is rather poor. For example, full screen is broken and some operations are accessible only via a swipe gesture.

If you've used NewPipe, how does it compare to SmartTubeNext?

[1] https://newpipe.net/

I haven't used NewPipe on Android TV so cannot compare, but the user experience of SmartTubeNext is top notch. Everything works (you can even use Chromecast from the official Youtube app on your phone directly to SmartTubeNext), you can customize it enough (show/hide relevant sections, default section on launch, shod hide shorts, color theme, SponsorBlock) and the features that I care about are supported. It's basically an improved version of the official app. The only thing missing I can think of is the lack of comments, that might be a negative point for some people.

Also I didn't personally interact with the developer but he seems like a very nice person, very invested in his project, even if some of the feedback he gets is thankless. SmartTubeNext is one of the few projects I'm actually considering donating money to.

I tried using NewPipe on an Android phone for a while, and concluded I wasn't in their target demographic. I understand, but don't like their choice of not supporting Youtube accounts. Also wasn't a fan of the UX or the settings, everything felt more complicated than needed.

No root required? I thought TV's didn't allow side loading anything. TIL.

No root required!

The installation process is a bit annoying, but updates work great.

That's great, it's a totally different thing to suggest to someone to sideload vs. to root.