Yes, it’s a lemmy thing that when the first personnel on your instance subscribes, it takes like a page of posts and then only the new ones going forward.
Lemmit.online used to have a way to add new subreddits, but it seems they have stopped accepting new ones. Must have got a bit too big to handle.
One warning: in my experience, you can not jump two major versions. Not just it won’t work, but that if you try it everything will break beyond repair and you’ll be restoring from a backup.
Two major versions can sometimes be a matter of a few months apart, so make sure you have a regular update schedule!
(Also, people say never update to a X.0 release, the first version of a major release often has major bugs).
TL;DR don’t take too long to update to new releases, and don’t update too quickly!
Also, the docker image is often a day or so behind the new release, soNextcloud tells you an update is available but often you then need to wait until the next day to get the updated docker image. I guess this is because (as I’ve just learnt) the image is built by Docker not Nextcloud.