So not whatever apps you want, but rather whatever operating systems you want? That makes more sense, but I'm skeptical that'd materialize to everything. On android phones with unlocked bootloaders (ie. "allow other software to run on the device if the user so desires"), and the only "other software" that's available are... AOSP forks. There are forks with better mitigations (eg. grapheneos), but they're still by and large, android.
This is mostly because making a different OS is insanely hard, at least if you want to support the many different pieces of hardware your OS might run on. Project Sandcastle is different and making progress[0], likely because Apple mobile hardware uses vastly similar components and suppliers.