Either I'm usually logged in when I move stuff or I have scripts that do it so is there a way I can just run mv something.zip /Backup/  and it will use the same rules as user-shares to decide on where to put it?

Either I'm usually logged in when I move stuff or I have scripts that do it so is there a way I can just run mv something.zip /Backup/  and it will use the same rules as user-shares to decide on where to put it?

unRAID will do exactly that if you move or copy files to /mnt/user/Backup  on the linux command line.  (everything under /mnt/user is the virtual union of all the disks, and will use the normal user-share allocation rules)

Perfect.  It doesn't use the cache drive which is good too.