I previously tried setting the permissions in each individual branch. I still had problems.
I took your advice and I am currently writing to a branch disk. It is working perfectly. It writes to a folder on the branch disk, then sonarr and or potato move it into the pool, in a separate directory (no conflicts). This is working perfectly.
So if anyone is reading this, it has worked for me.
I still may do a Raid 5 with three disks. It seems a bit less like getting it working with duct tape.
I will update if it stops working, just incase anyone finds this via a search.