i looked at volumino, lyrion and some others. didn't feel like i was really digging further into open source
i have a similar setup cobbled together with navidrome, mopidy (with subidy, MPD and other extensions) and snapcast running right along side it, allowing me to play two audio streams at once. (one is audio, other is announcement pipeline)
mopidy has a decent amount of dev action. navidrome even more so.
snapcast can sling anything across the network.
tied into home assistant for control and automations