Where to put the cap.
Put it right at the terminals on the PCB.
One side to the input, the other to the 0v - Shall we call it ground?
Logic 0v that is.
Don't put the cap from the input to the supply. That will make things worse, in all likelihood.
0.1 or 0.01 is OK. Either will do. I would go for 0.1 disc ceramic.
Try debounce 0, but 250 will be OK. If it needs to be non-zero, then find the minimum value that works, then at least double it.
Super X3. 3600rpm. Sheridan 6"x24" Lathe + more. Three ways to fix things: The right way, the other way, and maybe your way, which is possibly a faster wrong way.