On reddit fitgirls site is never mentioned in full for good reason.

That's just false. There's a direct link in the r/piracy megathread.

That's a problem solved by libreddit/redlib.

After googling, I think that was a joke about an unrelated app.

Lutris doesn't, but you can do firejail --net=none lutris

OBS supports capturing audio from a specific application, so if you set that up you don't need to be careful with foreign sounds.

I'd probably post quite a bit of content (music, etc) on a youtube alternative if I could expect it to not be copyright-striked. (and if anyone would watch it. The current PeerTube instances seem very barren.)

VLC, for playing videos and other media.

I'm surprised nobody mentioned this, but I use mpv for the same. It barely has an interface, but it's light and extremely configurable, and what I did was turn it into a picture-in-picture player (small, no border, etc, show subtitles if possible...). I use it to watch everything, including Youtube videos (it has builtin yt-dlp integration).

Or LibreTube, especially since Revanced needs Android 8+.

3rd-party Twitter clients are going to have quite a spike in popularity then, just need to implement clientside blocking. It won't prevent the person blocked from seeing your posts, but that's kind of impossible to enforce on a public platform, anyway.