Okay, so don't set up cameras in your house?

don't set up cameras that see public area. other than that you do what you want, but if a camera could see a neighbour's yard then they have a say too.

I've found on other forums that reolink can be set up without connecting to the manufacturer, and likely others. It's relatively trivial for experienced users to insulate any given device from the internet while using HA.

most IP cameras can be set up that way, yes. all you need is the camera to serve the video feed over RTSP, that's a direct connection.
but that's not everything. if you just connect it to your main network it'll connect to reolink servers without issues, and reolink can do whatever they want with it, including stealing the video feed, or if they turn greedy they can remotely upgrade your camera and disable the RTSP feed.

to prevent that, you should either create a separate VLAN for cameras, and configure your router (routing-wise) so that other networks (incl the internet) are not accessible from it. you need managed switches for that, or routers that allow you to configure VLANs.
alternatively get a dedicated dumb switch for cheap, and build a physically separate network for the cameras, and only connect the cameras and the server into it, without connecting it to the main network.

finally, what I meant with my first sentence in the last comment is that a passerby cannot verify your setup, and they shouldn't need to (or be able to) either. anybody can just claim "its self-hosted", so it does not really matter with respect to your neighbors and all the people who may pass by