Depending on your jurisdiction. Big asterisk.
Lemmchen
You're right, I forgot about the on-the-fly transcoding abilities of Jellyfin for a moment. But still, the server should be able to handle whatever codec they choose in hardware.
Keep in mind at least your server should be able to decode whatever you choose in hardware, so AV1 might still not be a good idea depending on your current hardware and upgrade plans.
With current hardware support I would advise against using AV1 or even H.265.
I'd recommend to experiment with H.264 and low CRF values to see what quality loss and file size OP is comfortable with: https://trac.ffmpeg.org/wiki/Encode/H.264
Yes, the only real drawback is the single channel memory connection, but that's rarely a bottleneck.
I'd recommend to go with some form of mini PC. If you don't need much CPU power there are some very cheap N100 ones where you can upgrade the RAM.
If you need E2EE there's only Matrix. I wish it wasn't the only option, but it is. For setting up rooms and so on you can use their Spaces feature, I think.
I've been looking for an alternative, too: https://feddit.de/post/8502516
From the ones you mentioned only Matrix supports full E2EE.
Does Forgejo have implemented federation yet?
Whatever floats your boat.
https://youtube.com/results?search_query=best+games+of+all+time
Oh, and go with Steam.
Apparently there are other issues as well: https://www.youtube.com/watch?v=T1KXeTd30KI