The most important part is getting the video. The popular open standard is RTSP. Many cameras have astonishingly poor RTSP implementations. See https://github.com/scottlamb/moonfire-nvr/wiki/Cameras:-Reol... for some of what I'm talking about. That ONVIF page (https://www.onvif.org/conformant-products/) says everything is fine for several Reolink models, but I don't believe it!
After that would be some way to get the analytics, assuming the camera has them and they're any good. If the ONVIF API works well, great, but a proprietary API isn't that bad IMHO as long as it's simple.
My opinion differs a little bit. If I buy a PTZ I know some here think those are a waste I expect to be able to control it from an open source application. If I can control all aspects of PTZ audio and video then that is good enough for me, but may not be good enough for others.
[1] - https://www.onvif.org/conformant-products/