

What filesystem are you using on the external drive? If it is NTFS or FAT, they won’t store permissions on the filesystem, which would explain why the owner/group changes are not persistent. To fix that, you can set the uid/gid on mount in your fstab.
/dev/mapper/YOUR_DRIVE /path/to/mnt <fstype> rw,uid=<jellyfin_uid>,gid=<jellyfin_gid>,dmask=0002,fmask=0113
This really depends on your threat model. If you are only concerned about the drive getting stolen, or wanting to keep the data on it private if you need to RMA the drive, mounting it automatically on boot with a key stored on the rootfs can be perfectly fine. If you are a journalist in a hostile country and protecting your sources from state level actors is a matter of life and death, then yeah, this would be woefully insufficient.