It has its own backend, but it can also use Invidious. As such, you can also run NewLeaf (an Invidious-compatible YouTube scraper based on the NewPipe scraper) in the background and connect to it via FreeTube, and that should work.
I would say Freetube, it's very similar to android alternatives but in Linux, can proxy your traffic trough individious' instances.. The only downside is the 1080p limitation
It's a bit strange because I've that I can play some videos @ 4k with no problems. And this limitation is here the most of the time, because you have to change the video format (mainly switch to webpm), and this format is not really supported by piped and individious...
There are solutions that work. Like you, I would enjoy a simple app that allows for subscriptions and playlists without tracking or using an account. I mostly use RSS feeds (newsboat) and watch the videos through mpv.
And yt-fzf is great too. It uses invidious instances instead of YouTube links.