@zappy for WP migration, https://docs.cloudron.io/guides/migrate-wordpress/ is maybe a good start.
Cloudron annoys the hell out of me if I/Cloudron doesn't control the DNS. Please allow me to make it stop.
Not sure what annoyed you 🙂 Happy to fix whatever that is, but otherwise, you can always change the DNS provider to 'manual' and then Cloudron won't touch your DNS.
Nextcloud limitations. There doesn't seem to be a nextcloud-dev version that lets me have plugins and do custom stuff without a ton of steps.
By nextcloud dev, do you mean just having a "playground" nextcloud installation ? We do this usually using "clone" functionality - https://docs.cloudron.io/backups/#clone-app . That will create an exact replica of your existing app into another location and then you can tinker on a non-prod instance.