Most people want open source firmware for their router like they want open source sewing patterns for their pants. Primarily these are consumer products where people buy the product as a way to intentionally avoid spending time dealing with the problem they solve.
Open source pants might sell well in a Hobby Lobby and an open source IP camera might sell well in a Micro Center, but neither would sell well enough to get shelf space in a Walmart... and the best selling one in Walmart sells a tiny fraction compared to whoever can land an Xfinity contract.
Those already exist: OpenWRT, OpenIPC. I wish more hardware was available though, especially for cameras.
I need surveillance cameras (outdoor mostly) but the cheap ones I found are a horrid proprietary mess. I even bought and returned one after the setup was complex and invasive.
Basically all I need is access to the live stream from a raspberry pi where I can store and process the video.
I don't, unfortunately. My suggestion would be to use cameras that stream over RTSP, an open protocol, so you don't need to use any proprietary tools/apps. Make sure the cameras are air-gapped.
This is not as "one and done" as you make it sound.