Q: I am leaning toward ReoLink's 12K cameras

 A lot depends on your lighting. Reolinks, as a rule, are not great at night vision, mainly due to limited config options. I do like their POE TrackMix model though. Two lenses, so you get a zoomed and a wide stream, plus the wide stream helps with tracking in the zoomed view. Better IR night vision than many of their models too.

Do not bother with high MP cameras. Even 4K is often overkill, but 4MP generally looks much better than 3MP due to the sensors involved. Cameras are focused on a targeted range. Adding pixels generally does not let you zoom in much farther, though they might give you a wider view at the target range. That target focus range is not always specified in the specs for fixed-focus models, but it can often be inferred from the DORI numbers. Autofocus can help here, but be aware that a bug or bird might cause the camera to refocus, and it might not focus where you want it to. So, it's best to get it to focus on the target area and lock it there unless it is a tracking model. Never try to cover more area than what your cameras can see without moving.

For choosing models, the same rules always apply. Sort the distances to the target for each camera and decide how much detail you want to see. Match that to the DORI numbers in specs. Then factor in the width of the view angle to cover that target area. Lastly, factor in the lighting needed. Especially for color night vision, you want to compare min lux values and see real video. Many push the specs by reducing the exposures down to the point of ghosting moving objects. Or claim with internal white light on. IR night vision is not as bad. In both cases, you generally want to run without the internal lights. Accent lights for color, or IR floods for IR, are the way to go to keep bugs from visiting your cameras. Oh, and they should support RSTP or, preferably, ONVIF, which most of them do. Ignore the NDAA compliance BS. Do not assume ANY camera is safe, and block them from accessing the internet.

For NVR, go with a PC and Blue Iris or Frigate. Blue Iris has an easier interface and has an extendable AI, but is Windows-only and $100 for up to 128 cameras. Frigate is free and runs on Linux, but can be challenging to set up for non-IT types. Both work with cameras that support RSTP or ONVIF.

Updated copy/paste for what camera or system should I get.

 While I love Unifi network gear, their cameras seem pricey and feature-limited.

For a doorbell, Reolink worked best for me (over Ring and Hikvision, which make most of the POE ones).

For other cameras, sort the specs to see the details you want in footage. Look for DORI numbers in the specs and ONVIF, or at least RTSP. Note that, as a rule, Reolinks do not have great night vision and offer limited image control. For color night vision, it gets tough. Many cameras, like Tapo, for example, crank down frame rates to brighten the image, which causes ghosting. Best is still Hikvision ColorVus, which will produce cloudy, day-like video if you can make out your hand in front of your face in the target area. Otherwise, if you can't see to walk without a flashlight or the moon, you want a camera that has IR for night vision.

Note: Avoid turning on in-camera lighting as it draws bugs and will white out close objects. Used IR floods or accent light as needed to add illumination.

Tracking is helpful, but it's best as an add-on. You want total area coverage recording 24/7 because no detection is perfect. Note: Reolink has a nice dual-lens tracking camera, but avoid the WiFi model.

NVR-wise, Frigate is free, and a lot of people seem to like it despite the setup pain. I tend to suggest Blue Iris for easy setup while still talking to most cameras and third-party integrations. I'd avoid all standalone NVRs. Especially Reolink's, which sets unknown passwords on the cameras, preventing direct access.

And of course, block all the cameras from the internet and the rest of your network. Preferably, only talking to the NVR PC.