I've been working on the later stages of the design, and have been trying to make the phone as repairable as possible. First, the major components are all modular - the screen, battery, CPU, and cellular modem are all on separate daughterboards that connect to the board with the keypad. This makes it so that repairs can be done in a matter of minutes with no soldering whatsoever. This also helps improve longevity because it lets you upgrade the phone down the line if you feel it no longer suits your needs. This doesn't mean much if it's hard to take apart, so I'm trying to find a design that's super straightforward to disassemble and reassemble.
Also, I've been thinking of a tamper-evident boot process that's as painless for the user as possible, while making sure none of the data on the phone can be accessed if someone installs a malicious firmware. Essentially, the user can pick a picture to be displayed on boot. It is added to the firmware image, and can't be read back from the phone. If the image is different from one boot to the next, then the user knows something has been tampered with. I'm not sure if this will be helpful at all, or if I should try to make something a bit better, but that's something I'll have to think about before the final product is done.