SQLite is a proper database. Realistically you’ll never exhaust its 278tb storage limits, it’s thoroughly battle tested, and it’s dead easy to backup.

I doubt nextcloud is running enough parallel db writes for this to actually matter — and if it is WAL mode is still probably good enough.

Once you have multiple software clients running then you will need a client server dbms like Postgres. For most home or group installations, this should not be an issue.