Just use whatever cameras and stick em on their own subnet that can’t go outside and only gets to talk to the server. Keeping them from phoning home is only half the battle. Ip cameras are inevitably gonna be riddled with security vulnerabilities no matter which ones you choose because they represent a set of libraries and tools frozen in amber.

There are no open camera brands to look for and you don’t get to harden the firmware.

That’s not entirely true but in terms of accomplishing the goal with a budget it is.

Use frigate or whatever. Consider not using zfs. Make sure to add a video card that can handle as many decode and encode streams as you plan to have. Your cpu will have its hands full shuffling data back and forth and serving the web interface. Something in ddr4 with pcie3 is fine as long as you can slap a 3050 or a380 in there. Consider having a separate physical device as opposed to a virtual machine on your extant server. In the case that you need to turn over footage to the authorities you don’t want to be giving them access to the futa SpongeBob memes folder along with the garden and garage surveillance folder.

E: a380, the 370 is a mobile gpu

https://docs.frigate.video/frigate/installation/

I think most PoE cameras work offline, but try to phone home. I use reolink ones and block thier internet access at the router.