I have a similar setup, 3 mini pcs (2 dell, 1 hp) with a second 2.5Gb interface on each and a synology nas.

Initially I used iscsi for storage but I had a couple of annoyances that I encountered:

• At that time, taking snapshots on vms using iscsi storage wasn’t possible.
• My Synology NAS doesn’t have a way to natively install a 2.5Gb interface, so I used a USB 2.5Gb NIC. Occasionally (like one or twice per year, from my recollection) the NIC would stop working and my VMs would lose their storage. Easily fixed by rebooting the Synology and VMs, but annoying none the less.

Because of that I ended up installing an SSD in each node and creating a CEPH storage cluster. No more issues with VMs losing storage and it gave me the ability to create snapshots.

My current environment is 17 LXCs and 4 VMs living in a ceph cluster that runs on each proxmox node. Using PBS, I have backup jobs scheduled across 4 days each week, 2 of the days backup to my nas and the other 2 backup to Backblaze.

I’ve never encountered any noticeable bottlenecks with my current environment so I think you’ll be just fine