So I finished my testing somewhat successful.
I divided that one and only /64 Prefix from my pesky ISP into some /80s. All my PCs, physical and virtual (Windows 10, Ubuntu Server 18.04) worked fine with that /80s, tested from inside and outside.
My older Asus-Router, configured as an AP, my Android10-Phone, connected to that AP, and my Dell MuFu-Printer, connected via Ethernet, stayed on IPv4 only.
Not the worst outcome I think.
So it is doable, but not for every device.
NPt with that ULA seemed to work also for one interface, for one other it wasn't at the time of testing, so I skiped this test, to later find out, that there was this bad static-route... So my conclusion, although not thoroughly tested, it would be working with ULA and NPt the same as with direct public IPv6 Prefixes. 👍