Im considering buying a new phone and i don't really consider a Pixel. I really like Fairphones approach, with the self repairable stuff. Even though they don‘t have a headphone jack. But well… I can’t change it. I’ll definitely go with the adapter over wireless headphones.
But to my question: What private OSes are there? Fairphone sells FP4s with eOS, how is that? And does it work on the FP5? GrapheneOS only works on Google Pixels right?
Graphene does only work on the pixel devices. What makes it special is that you can lock the bootloader again after installing it, which with things like lineage, you cannot do. I have never used /e/OS but i use lineage as my daily and it can be installed on FP
In that case, have fun coding up your own bootloader and flashing it onto the device. If you can't trust the bootloader, then you can't trust anything at all from the operating system that sits on top of it, because it could be compromised. If you can't trust a bootloader, then the only thing you can trust is a pen and a piece of paper.
Phones don't use an IBM-PC architecture. You'd need a phone based on an architecture phones aren't usually based on or You'd need to re-engineer UEFI to work for an architecture it wasn't designed for
I'd be more worried about the ROM that runs before the bootloader that you can't inspect, or possible hardware implants if you don't trust the bootloader shipped to you from the vendor.
This is unrelated. You want to familiarize yourself with the concept of OS integrity and how it is different from data encryption. You can have a passphrase that encrypts your data alongside having access to these hardware features.
It has very minimal code and its implemented in a robust manner. Unlike UEFI and the desktop implementation of secure boot, it does work well and it has not yet been exploited on pixels. Its way better to have any kind of OS integrity check than none.