My bet is they do considerably worse. Digital cameras are not designed with security in mind. Arbitrary code execution has been achieved on many DSLRs and there's even been open-source firmware projects for some.

Unfortunately they're a little outside of my tinkering budget, but if anyone wants to send me some I'll do my best to pwn them. Can't be any harder than a Google flagship, one would imagine.

I have ordered a faulty Sony A7 IV motherboard, but due to its faulty-ness and the lack of the rest of the camera, I'm not sure how far I'll be able to get with it.

Could you make use of a Sony a6000?

Why would someone paying for an expensive camera to damage the pixels of their images with "invisible" watermarks?

The signature doesn't touch the image data in any way. It's just a piece of metadata attached to the image, like any other metadata tag.