Based on your stated assumptions about backdoors, how about building your own RISC-V phone?

https://mikrophone.net/

It’s not in a very usable state, but the hardware is open source.

Sounds like you trust iOS sandboxing. I have my suspicions it’s not perfect, so how big of a deal it is depends on your threat model. If I’m on iOS I tend to manually enable the camera if I ever need it, and I also avoid using apps when possible.

If I needed to use an app daily that requires camera access, I might change my habit because that sounds like a lot of work.