Great tool for documenting your setup. I use this at work a lot

I would say that “on prem” defines a location, “selfhosting” an action. You can do both at the same time, e.g. selfhosting nextcloud onprem.

Apparently, it is not only my oberservation, but the article says similarly:

The inconsistent approach to backward compatibility in decades past may also have played a part.

However, I’m not a db admin and my perspective might be biased (infosec).

I would say that this is a sign of a bad product. Apparently, compatibility between SQL server versions is not great.

Badly. Nextcloud is a very active project with many plugins and integrations. You can even integrate a mail system and AI image tagging, chat and video calls.

Owncloud focussed more on the enterprise sector and less on fancy features. Definitely the more stable product (but not only in the positive sense).