Solved by following this thread: Issues upgrading 7.0.3 → 7.1. After this rollback to (updated) factory settings I could easily remount and use my external storage, and after installing LXC again I was able to use the already configured LXC’s on that storage. I had made screenshots of several settings pages that were important to me so after a bit of work I am now on Turris OS 7.1.2.