If you use the public instance you don't need to set up or host or install anything. You can selfhost it if you want, but the public instance works just fine.
One person goes to the web page and starts a room. The other can join the same room by knowing the name of the room. (It will generate a link when you create a room to make it easy to send to someone so they can join by just clicking the link.)
Consider giving MiroTalk a try. It has several versions but the P2P version would probably be perfect for your scenario. It's free, runs in your browser, doesn't need an account, and doesn't have time limit shenanigans. I've used it in lieu of Discord calls before and don't have any complaints.
Cocaine Sharknado (2025) I am so ready
They could create a new flag for Abandoned Early Access games. If an Early Access game hasn't been updated in a long time, that could trigger an automatic email to the publisher saying "Hey your game hasn't been updated in a long time and could be changed from Early Access to Abandoned Early Access. Consider updating the game or store page to keep Early Access status. If you would like to switch to Abandoned Early Access, you can ignore this message and it will automatically update in two weeks or you can manually change the status on your game's Steam page." Wouldn't really need more employees to handle this unless the current employees are all too busy to implement something like it.
I swear there was at least one more server I looked at but passed over and I cannot recall the name.
Maybe Jellyfin? It's best at movies/shows but it also handles music (and more). The native music experience isn't great but it works. For Windows/Linux/Mac you can use Feishin (I use and mostly recommend it, also you can use the web app version). Android has Symfonium I use and highly recommend it, also it works with FAR more than just Jellyfin). I don't use iOS but I just looked for an iOS app and found AmpFin (not to be confused with Finamp).
You said your users have their own libraries. Jellyfin works great with this. Out each in its own folder, create a new library for each in Jellyfin (pointing to each folder), and you can choose which accounts can see which libraries (and optionally let them manage libraries too so they can delete songs or modify metadata for the libraries they have access to).
I'm a fan of Jellyfin if you couldn't tell…
I use Watchtower and haven't had any major issues in the two(?) years I've been using it. Make sure you use persistent volumes for your containers and make sure you back up those volumes. If anything breaks, you can roll back to before the update.
If you don't use persistent volumes, you'll lose data when Watchtower takes down the image and replaces it with the newer one (which doesn't copy over ephemeral volumes).
I also recommend for database containers to use an image tag that won't update with breaking changes. Don't use postgres:latest, use postgres:15.2 or something like that (whatever the image you're using the database for recommends).
I didn't realize this when I first set up Radarr/Sonarr and they ended up copying every single file instead of hardlinking. By the time I realized, I had like 400gb of duplicate files. Ended up running fclones and getting it all back.
Portainer does store compose files though? I've manually used docker compose commands from the folders Portainer saves them in. They're labeled with numbers instead of project names which makes it difficult to know which one you're looking for, but I use rga so that wasn't as much of an issue for me as it would have been otherwise. It was tedious, but the compose files very much exist on your hard drive.
I started with Yacht and moved to Portainer. Yacht's ui was just too heavy and unresponsive for me. I got logged out of sessions without it actually telling me almost every time I used Yacht. I would have to log out and in again just to use it (a process that often freezed up as well for reasons I cannot comprehend). I finally had enough and switched to Portainer; not a single complaint since.
I had a few metadata issues with Jellyfin until I changed the primary metadata source to be the same as what Radarr/Sonarr use so they all the file names match up and I've had no issues since.
I also don't have a notable issues with subtitles in Jellyfin, but maybe your requirements have more friction. Have you tried the (iirc included by default) Jellyfin plugin to automatically download subtitles for your stuff? Or the *arr program that handles subtitles (I forget its name)?