When it’s 3 days away I’d worry less about bays and more about what happens when it hangs: auto power-on, a hardware watchdog, a fallback boot you can pick remotely. N150 boards with two 2.5G ports exist, but almost none take two 3.5" drives, so you’ll probably end up with a USB dock, and the neighbors will unplug it. Does it have to be one box? I’m building a small N150 home box and remote recovery is the part I keep going back and forth on.
I’d go mini PC plus a small UPS. A laptop battery sitting at 100% for years tends to swell, and most laptops give you one ethernet port, so you end up with a USB NIC, which is the first thing to flake on a router. A cheap N100/N150 board with two 2.5G ports and a $50 UPS gets you through the blips and leaves room for the media stuff later. Turn on ‘power on after AC loss’ in the BIOS. That one bites people. Disclosure: I’m building a small N150 home box, so I’m biased toward that size. If you go that way I’d like to hear what you’d want in it.
If nothing’s exposed and it’s all behind Netbird, you don’t strictly need it. I still would, because apps like Vaultwarden’s web vault won’t load without a secure context. A wildcard cert via DNS challenge is a one-time setup. Everything else is optional.
Put the cams on their own VLAN with no route to the internet and let Frigate pull the RTSP streams. Block DNS for that VLAN too, or they’ll keep trying to phone home. Curious what you pick for the NVR side.