If I could find a brand that actually did regular maintenance and wasn't a walled garden (such as Ubiquity), I would replace all of my camera's.
Security comes from networking tricks (VLAN's, etc), which is far from ideal.
Not pretty, not reliable, but surely more secure. Though the best thing is to air-gap or at the least firewall.
I wouldn’t discount network tricks, tho. A non-routed vlan for cameras to talk to the nvr and all video access through the nvr only keeps cameras pretty safe. Much easier to secure one nvr than 100 cameras
There's some various other boards we could possibly use. Few are small. Many lack CSI interfaces for attaching cameras. And then, for your outdoor use, ruggedizing is another huge leap.
Alas though, you'll still be stuck with a device that only lasts 3 years before it's insecure. I'm in the minority, but personally I'd rather a more open ended & flexible system like Linux, with more small-pieces-loosely-coupled possibilities in front of me, where-as with Android I'm going to have 2 or 3 different apps with pretty fixed/limited capabilities that I'll never be able to improve or adjust.
In some ways, the web is kind of the possible remedy here. If the phone runs a webpage that accesses the camera & webrtc & does the things, that'd kind of be ideal, because it's insta-deployable to any vaguely general-purpose hardware. Developing the webrtc chops though to be able to make use of this well though, that's a totally separate conversation.
You'd still have binary blobs of HAL for some peripherals but that's not different from raspberry pi.