For your stereo / audio zones, a WHA controller, integrated or preamp, will be much easier to manage than 30 A/B switches. If you want to DIY for cheaper than $50/ea, I’d suggest a bunch of DPDT relays.
For surround zones: Unless you need your intercom / paging in glorious 9.2 surround, take the trigger output from your intercom and send to your control system. Program that to mute the receiver when the trigger goes high, and use 1 A/B switch per surround room to send intercom audio to, say, the rear speakers.
If you really want intercom through ALL your surround sound speakers, you could also send line-level intercom audio to your receiver and have the control system switch sources, but better have some preset volume levels activate, too. If you have a movie cranked to 11 you may not appreciate the doorbell at the same level.