"in-house" "custom" of course it's difficult to support
I have used Linux as my desktop when working for myself. I found LibreOffice and it's derivatives to be a pig and switched to WPS (Kingsoft) Suite which works close to seamlessly with MS docx's. That's just a by the way, not really relevant to this case.
I can't understand why someone thought it a good idea to develop their own bespoke Linux build. Why not buy from someone who will support/update. In Germany, SUSE SLED would seem the natural choice. Similarly with the office suite, Softmaker maybe?
They would have current supported versions, someone outside the Munich Council IT team to blame for issues (yes, I've worked in public sector) and some cost savings to show against MS licenses. That's not maybe the open software big win but it would work sooner and save money.