What you miss with raw Debian compared to TrueNAS is compatibility. TrueNAS makes sure that all pieces are compatible with one another so that when you update each piece or OS, the storage doesn’t break. The whole package is tested throughly before release.

Also, TrueNAS makes setup painless: users, permissions, shares, vdevs, ZFS tuning, nice dashboard etc. With Debian, you get a lot of config files and ansible playbooks that become hard to manage.

Ideally you won’t run other stuff on a NAS, outside Docker.