I am similarly having trouble with Space Cadet (where Shift sends a parenthesis when pressed but is Shift when held). It interacts badly with other keys because

I am similarly having trouble with Space Cadet (where Shift sends a parenthesis when pressed but is Shift when held). It interacts badly with other keys because there's no way to tell it to never output the parenthesis. I need to be able to stop it because sometimes I tap both Shifts (right then left) to output Caps Lock or to output "()" (left then right). Since there's no way to tell QMK "I've manually processed what I needed to process from these keys, you can stop", it sends the parentheses anyway. It gets even harder to debug because, as you say, it's highly timing-dependent. Hold (tap) the key for 100 ms, you get one thing, hold it slightly longer and you get another. I have a todo item to reimplement Space Cadet myself one of these days so I can override it better, but I shouldn't have to go to such lengths. Other than that, I love QMK.