I guess you could try two ways - create a large filesystem on another disk (or elsewhere), integrate it into Nextcloud using the external storage app and move the content there, which is how I manage my 42TB storage array whilst keeping it available via other means (CIFs, SFTP, etc):
… or potentially also “move” your data dir:
“Move” because you’d be symlinking from the existing to new location. Pretty sure that’s supported with the snap install.
