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.