The motor work is great, but your 24-spoke back wheel is much too flimsy for motorization, IMO.
A lot of people have already found out that even using 32-spoke wheels, 16-gauge spokes are not strong enough, and many times 14-gauge spokes may not be either. People on motoredbikes.com tend to use 12-gauge x 36 spokes, or 11-gauge if they can get them.
And I would change the front wheel to a 32- or 36-spoke as well. To an extent this depends on how you ride, but a lot of high-end bike parts that work just fine at 15-20 MPH will not last long at 30+ mph, and the spokes are often the weakest link.
The spokes only break from the weight of heavy hub motors jarring the spokes over bumps, not horsepower. Look at it this way, most mountain bikes have 24 spoke wheels. I have one myself with 24 spoke wheels. If you have disc brakes on those wheels, the stopping power is handled by the spokes. However, have you ever heard of anyone breaking spokes from hard braking? Neither have I.
I even bought a 36 spoke, large hub flanged rear wheel for this project, fully expecting the rear spokes to object. However, I have 100 miles on it so far without any problems, and trust me, I have been riding it hard.
Also, the front wheel is not a problem. The brake was, though. I went to a larger disc for better braking.
I understand your concerns, but, as I mentioned earlier, the issue with broken spokes on e-bikes is the heavy hub motor puting huge tugging stress on the spokes from hitting bumps, not from rotational power applied to them.
I spent ALOT of time looking into this (after all, it is my bacon on the line if something breaks at 35 mph).