Herringbone Version

In this example, bearings should be added to planet gears to make the output shaft not wear out
Thus, this is not a turnkey solution for practical applications. Design must be modified.
This design appears to be completely stackable. 4x per stage gets you good geardown. Successive stages should be enlarged to handle higher torque.

This is good for a geared down extruder for 3 mm filament.
This may be applicable to vehicle geardown with rubber-on-plastic drive for heavy duty applications up to 1000 lb drive for solar slow traction vehicles
Modification:

add bearings to planet gears
add a cover with bearing. Planet gears serve as second bearing support point, but a double bearing in cover would release pressure from plastic gears. Because of short profile, we can add double bearing in cover to get lifetime >>1000 hours for extruder geardown applications
Use hobbed bolt as the integrated shaft

Straight Gear Version

This one has bearings, so it appears to be usable as is. No herringbone gears, though. - [2]
May need to increase cover plate and double up shaft bearing to address radial thrust in filament drive application.
Easy fix, outside of getting bolt lengths correct.
Disadvantage - there will.be wear as axial thrust is not addressed without herringbone design.

Note that herringbone gears eliminate the need for a thrust bearing. That is a great point of size.plification when using planetary gears.
"Here is a design for a low-profile (18mm) stackable gearbox that fits onto a standard NEMA 17 stepper motor. The gearing ratio of 4:1 allows for increased torque and positional accuracy from the motor. Herringbone gears are used to provide smooth power transmission from the stepper motor while eliminating the need for thrust bearings. A flat rotating output platform is also provided to allow for easy mounting of attachments. More details along with a step-by-step guide to designing gears for 3D printing in Autodesk Inventor is provided below."