I would go with Jellyfin as a server and Mopidy client as outputs. Like an rpi with an audio hat or class d amp hat. Planning to add Snapcast to the mix for multiroom audio but didn’t find the time yet.

mp3

avc1 is the name for H.264 (also called AVC) which is admittedly pretty confusing

MP4box has a great local web tool to test the mime type including the codecs parameter:

https://gpac.github.io/mp4box.js/test/filereader.html

Jake Archibald wrote a good article about this topic: https://jakearchibald.com/2022/html-codecs-parameter-for-av1/