My favorite way to keep the shaft in place is to have the shaft be hex below the bearing and ThunderHex through and above the bearing. If you use a ThunderHex bearing, the shaft is captured by the step from hex to ThunderHex. This is what the SDS modules do.
A clamping shaft collar above the small bevel gear would probably work too if that were easier (in place of our in addition to spacers). I generally don’t like collars for anything critical, but there wouldn’t be much upward force on the shaft itself (just on the gear) so it probably wouldn’t slip.