[Joe G.]

Good start!

What was the rationale for the split plate structure? As drawn, you don't really gain any practical strength from the outer plates, since they do not constrain the shafts in any way, or reinforce the parts of the inner plates which will see stress. Tolerance stackup between the inner and outer plate may also cause you issues with gear mesh at the miniCIM pinion, depending on how it is done. The only thing I can see that the outer plates are doing for you is standing off the miniCIM so that it doesn't interfere with the 1st stage shaft's bearing, but this could be achieved through either a thicker plate or a standoff plate slipped in front of the miniCIM.

There's a lot of material up top that isn't really serving any purpose. This can be narrowed up a lot to save some weight. A cool trick which could come in handy here is to run the motor mount bolts all the way through both plates, with spacers serving as your upper standoffs.

It will probably be fine as is, but it's generally considered good practice with this kind of lightening to put a band which runs all the way around the outer edge of the miniCIM, so that it has a solid surface to react against to prevent bowing.

You're missing a few fillets in your lightening, which will cause problems in manufacturing if this is made with a mill, and will generate stress concentrations where you least want them no matter what.

How do you plan to retain the output shaft?