I think saying "RTFM" will be a waste of time on this client- this isn't going to be an easy system to operate for someone who's not a tech-head. I would can the BT speakers and use something easy, like Sonos (even though I don't like their BS marketing tactics)- I like HEOS more, but it's not always as easy and in a complex system, I would expect O'Toole's Corollary to Murphy's Law to show up ("Murphy was an optimist"). I would also limit and compress the crap out of the audio in order to prevent killing speakers.

I don't think there's any way to eliminate delays if the direct sound of the instrument(s) can be heard in other zones unless all doors and windows can be closed to block the sound.