Nice. Volume controls work, artwork and sound info is good.
A few things I noticed
The state remains at idle when streaming internet radio.
There is no information on what source 1-4 the zone media_player entity is attached to.
In the discourse: https://amplipi.discourse.group/t/home-assistant-temporary-rest-integration/77/3
They created a universal media player with the zone's attached source as the state of the zone. So far this seems to work for me along with using their rest commands to change which source the zone is attached to. Perhaps there is a way for you to use the join/unjoin service for media_player that could work here?
This issue is to track item 1 from this conversation.