@maiyannah You can decide which port you use in the SRV record as well ;)
All !XMPP servers should obey the SRV configuration.

Also: It's the ID that can't change, not the SRV records. And you can put SRV records for subdomains too. It's sort of like SNI - except with your ID. When you connect to a server you tell it which host you want and which local user you want to identify for.