LonelyNematocyst

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.

Staying on your home instance is currently an issue on the repo. Implementing it yourself would need using the search, which is complicated because it only works through a websocket connection.

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.