If you need more torque than a harmonic will handle, look at cycloidal drives, which can also have zero-backlash. They're used to power the joints in robot arms, which have to hold absolute position or move strongly against cutting forces. They're more mechanically complex, and hence more expensive, but you might be able to find used ones. Here's an article that explains some more about them: https://www.motioncontroltips.com/cycloidal-reducer/. .