I think you can do push-to-talk/drop-in at least via tts using BroswerMods on home assistant, that would be one option.

Navidrome and Gonic are very active projects yes. Why would it not be a thing anymore? Works fantastic.

You can use the DNS verification method. Either using nsupdate with bind or what ever protocol your DNS provider and favorite ACME (certbot, acme, lego, etc) utility supports. As long as your DNS server is publically reachable that will work, even if the subdomain itself doesn’t exist publically.

Have you never heard of OpenStreetMap?