Schpankme,
I agree to Steven about this. You can imagine the magnetic flux as a liquid or water and the core material as a water pipe. You can see that in the case of your motor, the poles are the thinnest parts, that is like there is very small diameter pipes. That will cause some resistance to the flow.

Anyway, today I was at this motor winding company and got my silicon steel plates. The plates are 0.35mm thick and I bought about 18 kilograms of the steel, that should be more than enough for a 120mm diameter core with 60mm thickness. Here's a picture:


I paid about 15$ for these plates, I think that this is very cheap
Also I saw the whole winding process and it is exactly like Steven said, they wind those coils on a separate machine and only then put them into the core:




Also saw some pretty impressive size motors there, this one for example is a 75kW motor:


They have a lot of smaller motors there, but they all have the same type of stator cores and nothing of what they have is suitable for our purpose.




So now I will design the motor in 3D and see how it looks. And then off we go to the laser cutter