Actually the largest is 10mm. (unless you have a larger on custom machined, which is technically possible, but it probably wont last long)
The shaft size itself doesn’t necessarily determine the load of the bit.
You can load up a 1/4" shaft bit by reducing Spindle RPM. increasing Feed and Depth per pass enough to cause damage so I wouldn’t necessarily say the shaft size was selected as a result of limitations of the machine… maybe as a deterrent to reduce unintended mis-use by a new CNC owner, But If used properly, I’m sure the machine itself can run a larger shank bit… could you take advantage of that larger shank to apply a higher load than with a smaller bit, probably not, but this goes back to the prior sentence about the shaft itself not being the single factor that determines load applied.
As large as you can get in a 10mm shaft, with respect to appropriate cut parameters.
Most Likely No, But I’m not a representative of the company, so take that as you will.
In my opinion: No; the next step is a more rigid frame (specifically wasteboard supports), followed by direct drive ball screws (get rid of the belts from stepper to ball screw), followed by rack and pinion (even more rigid and accurate), Then a larger spindle would be in order which could properly support larger tooling and higher applied loads.