I could not let go if it, so I kept digging a bit. I had previously reset the memory limit back to its default, but bumped it to 6gb now for another test. This time the restore of Nextcloud went through. So now everything is operational again (and I also cleaned out some apps that had errors, but I was not using anyways anymore).
To get back to a working state I needed to restore each up using a postgres database to the state from before the upgrade to 8.3.0. Else the app would have no data in their databases (Mastodon users and toots gone, Matrix rooms gone, for example).