Have you looked at OpenTorque actuator? https://hackaday.io/project/159404-opentorque-actuator https://hackaday.io/project/159404-opentorque-actuator Not sure

Have you looked at OpenTorque actuator? https://hackaday.io/project/159404-opentorque-actuator https://hackaday.io/project/159404-opentorque-actuator Not sure how appropriate the form factor is for you, but maybe you can at least find some useful ideas for your own gearbox design.