I finished the build one week ago. For me it's a masterpiece. There is so much detail and also these modularity. But...
I think that the not so good working gearbox is the price for this kind of build. I have the same problems like all other that you can drive without the body but not with him. I tried everything from 1 to 3 rubber and nothing works. There are 3 problems in my eyes. First, the switch between 4 and 5 has no zero point, its a question of luck that the switch will fall in neutral. Second, the coupler is not able to handle the weight of the car. Most of the time especally in 5 the driving ring in the coupler will fade out from gear an the mechanic is not able tho resist. Last, there are to much dead gears. In 1 you have eleven gears to drive without need. This brings a lot of resistance to the motors.
Anyway, i have no idea to make it better :-) From building view it looks perfect. I think this car reaches a border between the used technic and the usability in relation to its original.