Dual nic NVR then? You could even just throw a simple switch with no uplink (but preferably managed so you can tag the traffic) and for extra safety just allow only the LAN traffic you want on the NIC/Port connected to your regular LAN from the NVR.
Nothing wrong with a DIY can though! As long as it works of course
Yeah all of my servers are on usff PC’s, so I get it.
If you do a hypervisor like proxmox, then throw your NVR in a VM, you can just create a couple of virtual NICs (though you’ll be back at that FW issue I’m sure).
USB NICs are pretty well supported these days though, and cheap to boot. Just need to be certain you’ve got usb3 if you want to make use of that gig though!
I’ve got a few pi-a-likes that I’m doing similar camera fun with, though using some webcams in there and a 3d printed case. At least that way they match my diy temp sensors with esp32s!