Pretty soon you can start charging to tune melodicas! I hope to be able to tune my own at some point in the future and will have lots of questions for you when I gear up for tuning. Right now I limit myself to cleaning (to get rid of the attic smell in used melodicas) and adjusting reed gaps for performance improvements.
Good point about re-tightening the screws with a craftsman’s skill. If you just jam a screw in a hole and start turning them with a screwdriver, you will eventually cross over the plastic threads too many times. Eventually, the screws will not hold. I do the same thing you do. I rotate the screw backwards until it rises up and falls (clicks) into the original threads. Then I tighten the screw just snug. The screws screw in nice and easy this way too.