That is the exact same breakage pattern I experienced many times. I don't believe there's any way to get more tooth engagement because of the way the planetary gears are set up. The single sun gear interfaces with the lower half of 2 planet gears, and the upper half of the other 2.
I wondered about flex in the plastic diff case but I had the same failure when using aluminum diff cases as well. Those planet and sun gears are just too small for today's brushless motors.
If you want to try adding something to slow the diff action down, I'd suggest trying diff putty. A small amount of diff putty, while much less precise than a given weight of oil, has the huge benefit that it won't leak out. Any reasonable oil weight (20k, 50k, 100k) will leak out of that diff unless you perform some surgery to seal it up. You can try super heavy oil, like 1M weight, to see if that gives you the feeling you're after, because such a heavy oil won't leak out very easily.
Good luck! Part of the fun is experimentation and seeing if you can solve these problems. Maybe you'll come up with a new solution that will help others.
Last edited by smirkracing; 09-07-2022 at 10:48 PM.