That's exactly what I did for years, not really with the "ch" sound but with ones that start with "t" or "th," sometimes "ch" though, but like you said, anything starting with a hard sound that I felt the stutter incoming. Now it's so subtle that no one notices. It felt weird and wrong at first lol but it felt better than stuttering. Too bad my speech therapist never mentioned that, had to figure it out myself lol. I don't fault her though, she was school-appointed and was helping multiple students at once, not really one-on-one. I think it's a great tip.