Which NON CHINESE smartphone brand is easier to tweak it's kernel?

I know most of the world's electronics are manufactured in China, but I am planning to keep my projects as free from China as possible.

I am planning to develop a mini Helicopter for the major project, in collaboration with my team. Now, I have almost 3.5 years to submit it as my final year project, so I have started to discover what's possible and what's not!

Recently I broke my phone, and in panic I studied almost all the electronics of a smartphone, and suddenly it clicked me when I was watching this video where the guy builds his own phone from a broken Samsung Flip, that I can use my old phones without the screen as the main computer that can run the helicopter, which will significantly reduce my R&D time on MCU(previous plan, also in consideration).

So I just wanted to ask which NON-CHINESE smartphone brand is easier to tweak its kernel?

Initially I was planning on printing a PCB, not from PCBWay of course, but maybe some local printers here in Mumbai, and mounting SMDs, like MCU, IMU, RAM, and loads of sensors.

Now the issue here is that I am trying to prototype a helicopter, that the NSG already use.

Its a compact design with 15cm length, 7.5cm height without blades and 2.5cm width. With these dimensions, the most challenging part is compactness.

What would you guys suggest?

a) Going for a complete microcontroller(self designed) or

b) Re-packing mobile SoC and sensors?

If option B, then which smart phone to choose?