one alternative would be to have a phone with a cpu that can be desoldered and replaced easily and in a more voluminous case (maybe a nice cnc milled alumin(i)um case)
"Desoldered and replaced easily"? Clearly, you've never actually desoldered anything. :) It's a tricky process which requires a good deal of special-purpose equipment and training, and even with that, there's a nontrivial chance that you'll damage the part, the board, or both in the process. It's not something you'd ever want to do on end-user equipment.