

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.
I have not deployed Garage S3, but it has a static pages feature you could use — just buid your static files with jekyl or something, create a bucket and set the permissions.