To fix big transmission gear on transmission threaded rod use "Option2" in assembly video on our YouTube channel (starts from 5:02):
We do not use sleeve with fixing screw anymore. We use threaded rivet and M5 nut to counter it instead. Screw rivet and counter nut towards each other to prevent them self loosing.
Also you may need to put just one M5 washer between wheel (threaded rivet) and ball bearing to align big and small gears. But it's not significantly. You may put or not.