Yes, I've talked to the developers about GrapheneOS. Unfortunately I don't have Pixel phone, the only one that GrapheneOS supports. It is intentional, because of some features of Pixel.

I use LineageOS, but if you want you can use crDroid, Arrowos, and if still exist e.g.: AISP, ParanoidAndroid, and so on. All of them devived to the CyanogenMod, the first naked AOSP based custom rom.

Or you can use the simple AOSP if you have enough time to build it to your phone.

On the other hand you can use some kind of Linux (anno there was e.g. UbuntuTouch)

Unfortunately Apple is a closed system and your only chance to escape is a "jailbreak"-ed iPhone (i haven't use iPhone for a long time, so I don't know if it is the correct name when you can access the core of the iOS). It is similar to "rooting" the Android