Not convinced it’s the right solution yet, but I have it working how I want it, at last.
I store my photos in Immich on an NFS mount.
I applied storage template that stores by album, then created another NFS export for that folder which I then mounted into the ./uploads folder within the MMM-ImagesPhotos module.
Because I’m running MM within an unprivileged LXC container, NFS mounts from inside the container directly are not possible, but this worked for me: I’m a link (essentially mounting to the Proxmox server, then using a bind mode to expose to the container)
Everything seems to working OK so far.