This is a plausible and often used setup - separate “disks” for the system and data…
I haven’t used Proxmox (my understanding, some virtualization tech).
But normally (when doing it manually), I’d install Nextcloud on a Ubuntu VM.
And before starting the last web-based part of the setup, mount an external disk for data.
When running the setup, point to this disk…
If you are going to mount this disk over CIFS, it should work.
I have only used ESXi and NFS/iSCSI storage, have no hands-on experience with CIFS.
Make sure your network is fast enough and not too busy…