Harmonic gears aren’t good for back-driving, as you’ve found out…
Since you need a reduction for the torque, you could take a look a cycloidal gears, these are generally back-drivable, and yield high gear ratios, but mechanically a bit complicated and hard to make compact.
Planetary gears are probably the classic way to do it, and you can in fact buy pancake style BLDC motors with integrated planetary gearing. These are sometimes also called “quasi-direct-drive” actuators. Unfortunately they tend to be on the expensive side.
If you can change the orientation of the motor by 90°, there are also in-runner BLDCs with integrated gearing and also integrated encoders. Maxon, Portescap and Faulhaber would be companies that produce high-end products of this type, but you’ll also find less expensive models.
Since you’re already ordering custom motors from China I’m sure you’ll find what you need 
You may also be interested in capstan drives.
Please let us know how it goes, it sounds like a very cool project you’re working on!