I have been messing with getting jellyfin working on the truenas side as well and your post saved me after 3 days of messing around and learning about datasets and permissions. I just want to add one thing, if you have your media in their own folders like me you will need to hit the Apply permissions recursively, hit confirm in the pop up, then hit Apply permissions to child datasets. I am still just learning and messing around with this so if there is a better way feel free to add your input.