An update on the whole boots in emergency mode situation. I am pretty sure I made lots of mistakes, but here goes:
After saving all the data I needed, I decided I could not risk an even worse failure. I installed OMV 6.0.24 without a hitch, it even recognised my RAID array as such and was able to mount the file system and folders the way I had them before. So all good right? Well, not so much. When I went over to services, the system yelled at me that there is obsolete data cached. Not knowing how to clear the cache at the time, I rebooted the NAS system, whereupon I again was greeted by the failure and dependency messages from my original post. It was time for Scorched Earth. I wiped both 4TB drives, uncoupled the RAID array and set it again. This took about 18 hours of thumb twiddling. I created a new file system and tried to mount it, only to be shown a message that the structure needed cleaning, similar to this thread. Emboldened by the simple looking solution, I happily started PuTTY, logged in as root and entered
fsck /dev/md0
There were nearly 29000 blocks with wrong inodes to clean, so the whole thing took a while. Please do not ask how I confirmed every single question the system threw at me. Afterwards, I was able to mount my system just fine, only to stumble on the next thing. Or the previous thing. The services were still storing obsolete cached data. I then found this thread, with an embarrassingly simple solution. Well, you live and learn.
Now my system is up and running, all photos have been copied and are accessible via the Windows Explorer, as well as from my Android phone (after some tweaks in the Windows settings). It is great for uploading the hundreds of baby photos this server was actually built for.
In the end I did not really fix anything, just rebuilt from scratch. I still have no idea why the problem occurred in the first place. I guess I will have to set up a backup backup location. Applying fsck before I had wiped the drives might have remedied the situation... then again, it might have not. Now I go put The Lord of the Rings on my Plex again.