Multiple buck converters and their draw
I can tie all of the grounds together (of the µC, USB-C and buck modules)? In that way, they all share the same reference and it means logic to the pixels will work

Yes

Does that mean that if I draw 10W on a buck module, the module will draw 12.5W from the 20V supply, or 1.6A?

Yes, for the 12.5W. But
12.5W/20V = 0.625A