Hello,
I found that monster of a brushless DC motor: https://www.gobilda.com/5202-series-yellow-jacket-planetary-gear-motor-188-1-ratio-30-rpm-3-3-5v-encoder/
As you see, this thing got a ridiculous torque, and it even got an encoder! So, I was wondering if someone know a book or a tutorial about making servo motor from dc motor. I never tried to build my own, but having a smart servo of this strength is a tempting idea, especially when considering the price of other smart servo with similar specs.