I agree Nik, it’s best avoided if possible. I think I assumed that the System Photo Library was always linked to iCloud photos but obviously not. It’s a while since I’ve used any additional libraries although I do have a couple of archived ones I might try and open.
Presumably, one could also do it through a VM? I have Mojave running on VMWare in order to use some apps that don’t work with Catalina. You wouldn’t even have to switch users then.