I'm assuming this is (one of) the threads you were looking at? https://forum.proxmox.com/threads/tutorial-unprivileged-lxcs-mount-cifs-shares.101795/
I know little about proxmox/LXC but lots about linux and containers in general so I might be able to help
Seems like you should be able to go into the LXC container and run (as root user, or with sudo):
groupadd -g 10000 lxc_shares
usermod -aG lxc_shares jellyfin
then shut down the LXC
And then on the proxmox host run (also all with root):
mkdir -p /mnt/lxc_shares/nas_rwx
echo '//NAS/nas/ /mnt/lxc_shares/nas_rwx cifs _netdev,x-systemd.automount,noatime,uid=100000,gid=110000,dir_mode=0770,file_mode=0770,user=smb_username,pass=smb_password 0 0' | tee -a /etc/fstab
Where //NAS/nas is the IP or hostname of the share (possibly localhost?) followed by the share folder name, and the username and password are set appropriately
mount /mnt/lxc_shares/nas_rwx
and if that succeeds,
echo 'mp0: /mnt/lxc_shares/nas_rwx/,mp=/mnt/nas' | tee -a /etc/pve/lxc/LXC_ID.conf
replacing LXC_ID with the VM ID of the LXC container (not sure how you get this but if you only have the one you could probably ls /etc/pve/lxc
and see what options are there
Then you should be able to start the LXC back up, and when you get to the point in jellfyfin of setting the media folder, just type /mnt/nas and press enter (or a subdir of it ig) and it should find it
if you did all that and it didn't work then would need more info to help troubleshoot