iPhone shouldn't even be on this list. Its a nonstarter straight out of the gate when it comes to installing a custom ROM. Whereas Android (the operating system) is open source (AOSP or Android Open Source Project). If you have an open source iOS repo you can show me where you can modify the spyware out of it and harden, maybe it can be considered, but in its current state, its a nonstarter.

So of the devices that can run a spyware removed version of Android, why Pixel? Because Pixel is the only hardware that allows users to lock the bootloader after they have unlocked it to load a custom OS. That's it. That's the reason. If you have other hardware that can run Android that allows you to do that, it should be taken into consideration.

Now all of that said, Linux phones that are not Android (Android is a fork of Linux and Linux is a fork of Unix) do exist and if you feel so inclined, you should look into them.