I now have a full size servo controlling the shifter. It's so much stronger than the micro servo I tried and doesn't heat up at all. I think that issue may now be resolved.
I also put this stuff in one rear axle to try out. It looks like it'll work better than the 2.5M weight oil I was using and shouldn't leak out. Now to do the other rear axle. No need to do the front axle, though. I have something different planned for it!
I'm also starting to make some progress with the ESP32 MFCU. I have the steering worked out, but that's all. I still have quite a bit of configuring left to do. Having to also learn a new transmitter just adds to the complexity!