Homepod Mini "no peer to peer" warning
...and just like that I found a solution. IGMP snooping wasn't enabled in the LAN bridge, so AirPlay packets were not being routed correctly between wired/wireless devices. I didn't even consider that the LAN settings would be part of the problem, as all devices involved were wireless. Unclear to me whether the mDNSresponder package is required for the router to make this work, but it's working now.