Thanks for your coturn write up. We did refer to that when we added turn support in Cloudron 5.1, in fact.
On how to update: you can just uninstall turn server (and also free up the turn ports on the host). The turn server is exposed at my.domain.com, so no need to setup an extra A record. The nextcloud talk app is pre-configured to use the integrated turn server. You can just "re-configure" the app (for example, just go to Location and click save without making any changes) and it should overwrite your old turn settings and put the new one.