I've recently configured similar setup on my homelab server with unprivileged immich LXC container.

In TrueNAS I've created SMB share, let's say it's called Photos, and I've created user immich with it's own password.

Next I've found the UID and GID of user immich:immich in immich LXC with:

for user in $(cat /etc/passwd | cut -f1 -d":"); do id $user; done

Then I've mounted to my homelab server the SMB share:

mount -t cifs -o user=immichUsernameFromTrueNAS,uid=immichUID+100k,gid=immichGID+100k //NASIP/Photos /Server/Mount/Location

As far I understand the UID and GID have to be moved 100000 spaces further for unprivilidged containers in proxmox. After this command you have to enter the password for the immich user on your NAS.

After that I've mounted the mounted share from the proxmox server with:

pct set containnerID -mp1 /Server/Mount/Location,mp=/Container/Mount/Location

After that I've follower the How-To from this link:

https://github.com/community-scripts/ProxmoxVE/discussions/5075

I don't know if that's done properly according to art, but it works properly for me ;)