The only deal-breaker for me was that the android app doesn’t persist its play state, so if I pause and do other stuff on my phone, it usually loses its place.

There’s ‘finamp’ for jellyfin which I really like so far.

I haven’t used plexamp though, so I can’t vouch for it as an alternative.

I use gnome-session-inhibit quite a bit, but it’s hard to imagine a good way to automate it.

Sometimes I inhibit idle to keep something on screen, and sometimes I just inhibit suspend so something can complete.

It probably doesn’t make sense for the terminal to have anything more than a protocol to control it. The only real benefit to that would be in remote sessions, and it’s not really clear how it should work when multiple machines are involved.