Somewhat late reply, since i didn’t have the time to start my conversion to OMV sooner. Definitely looks nice and installation was easy.
You are spot on that my first issue has partially to do with permissions. Following a guide at the time i structured my media library to some level with data sets in my zfs pool on truenas scale. But those apparently are more like filesystems rather than just directories. Which makes the whole giving permissions for my dockeruser a bit difficult (and also otherwise seems a bit less then ideal for OMV).
So since i wanted to structure it slightly different i made a new shared folder on my zfs pool and directories for my media in there and moved a few test files. Downside now is that this solution would mean that i have to move a ton of files, which would mean a ton of writes to my drives.
Apparently they changed that in one of the recent updates, but so far i really like the setup with docker compose. So much easier than the whole deal with applications in truenas scale. Copy+Paste stuff into a file, change some variables and a full stack of media applications is up and running so fast.