OpenBSD on an old office PC I picked up from University surplus.

You don’t need an NVR. The camera supports a microSD card to store footage. It also supports FTP upload.

HomeAssistant isn’t required. It is supported and I do have it configured.

I have next to no recollection of how UniFi works so I can’t speak to that. Mine is connected to a 2.4GHz WiFi network. IIRC that’s all it supports. Might be different with newer models.

It also supports HTTP(S)/RTMP/RTSP/ONVIF streaming. Though I tend to just use the app (and I’m particular about installing apps) as I find it quite usable.

I bought a Ubiquity router once because a friend was raving about it. It crapped out on me after maybe 6 months? Will never buy their hardware again.

I’m using a Reolink doorbell camera and am happy with it so far. Had it for about two years.

I have a PC that I use as a router. It’s power setup is inverter > lifepo4 battery > battery charger. Relevance here is that it’s a “dumb” setup insofar as the PC can’t manage the battery.

What I did to help with battery health is figured out how long the battery should run the PC and have a scheduled disconnect of the mains power for about half that time once a day. Not as good a solution as something that can maintain a ~80% charge, but better than leaving it at 100%

It’d work for an old laptop too, and can be accomplished with a cheap aquarium light timer.