or you could slap a GSM shield on a Raspberry Pi.

As I wrote: that's a MVP, not something you can sell to anyone less nerdy than Richard Stallman, and it's based off of the work of a lot of the people I just spent 58 minutes to think of and write down.

Then you can copy the relevant parts of the designs from both boards onto one smaller board.

Good luck getting that to perform. Or even boot. High-speed buses are the darkest of the dark arts to design, at the kind of frequencies particularly PCIe runs even the slightest trace length mismatch, impedance issue with the PCB itself, vias, or even external RF sources can, do and will mess with your sanity.

That's my entire point. It's not easy to design a complex thing such as a smartphone.