Cannot rename Pictures folder
If you are not using iCloud sync - game on. Hard link / sym link - unix filesystem is a unix filesystem.

If you are using iCloud, it intercepts the filesystem IO at a low level and I try not to mess with the folders Apple makes In this general case since as soon as you update the OS or some software makes assumptions about what folders exist, then things break.

I love storing my main photo library on secondary storage so you’re on the right track there.

What’s stopping you from making a pictures folder in the Documents folder and just using that? If I understand you have a Pictures folder as well already in iCloud so just updating your finder sidebar and recent locations to be there would get all the apps to use the new cloud pictures. Maybe I’m just not seeing the case where something can’t find your new Pictures folder or what’s failing in your mv command or attempts to remove the user folders.