One thing I've found useful for me working on a large project is keeping all of my global variables in their own event sheet and separating them into sections with comments/titles.
This sounds like a good idea, but I'd add another suggestion which I think is an under-used approach in Construct: if you need a global variable, but you only use it within a particular group of events, then make it a static local variable instead. Being a local variable means it is only available in that group, so it doesn't clog up the global variables list; being static means it remembers its value permanently like a global variable.