Nice little machine. I just sold mine.
I had upgraded mine to 12 GB (8+4 non-ECC). I see no reason why you couldn't go up to 16 GB.
Why ? 2GB is fine if you're only using it as a file server, but if you are using your NAS to run multiple VMs, then the extra RAM can come in handy. I ran OMV, Plex, Transmission, Sonarr, Nextcloud, Pi-Hole, Logitech Media Server, Jeedom, etc... all in different VMs under Proxmox.