I must be super gentle to watch when its about as flat as the surface, maybe a bit more inside, and stop tightening, otherwise I tear up the wood.
And thats not a great solution for skateboard with vibration problems and stuffs like that.
so my question is:
Is there sort of special washer for such screws to avoid that problem?
I was thinking about countersunk washers like this:
First what type of wood did you make the board out of cause hard maple should not compress that easily unless you have a 1000 lbs gorilla doing the tightening. I would just suggest the x brace. Counter washers I think would be to large for it to look nice. Fender washs may do the trick but you since you have a large hole it would sit over you might cup the wash.
I would do the x brace or get pan head screws with a flat washer. What you should do is drill a countersink hole before you put on the hardware. Doing this will create an area that the screw head can sit and not compress the fibers of the wood that could cause premature failure of the board .