[4DThinker]

Your patience is amazing. What you might also add is something that turning one gear set moves, such as clock hands or an iris (think old fashioned camera gear) mechanism. I grew up with Erector set kits given to me at Christmas time several years in a row by my grandfather (an architectural engineer). The joy in them was that after playing with them you ended up with (usually) a working toy that also was fun to play with. They also encouraged my imagination to make my own designs with the parts. The lasting box-of-parts toys typically had a destination reward for succeeding to put the parts together in a certain way.

4D

 

[4DThinker]

If marbles are used in the gear set how about making a couple of marble gears? Not sure how well they would work, but instead of meshing teeth make coves in each so that only when a marble drops through it engages mating gears and gravity spins them. Or glue marbles into one gear that ride in coves on the mating gear. Let the marbles serve as the teeth for the gears. Or every other cove in two mating gears has a marble glued into it.

Sorry, but once you said "marbles" I've been possessed by the possibility of marble gears.

4D