She's using Lightroom classic right?
After importing from the SD or CF card, Lightroom stores the original imported photos in a YYYY\YYYY-MM-DD directory. That's usually where the bulk of the disk usage comes from.
Personally I just move those archived photos older than a few months to external storage and re-map in Lightroom.
I dumped synology and just use proxmox for the automatic ZFS support, then I can run my apps in either containers or VMs and even do GPU passthrough if needed.