I'm trying to get onstep to work on a planetary gear based 3D printed EQ mount like the one linked below. The board will be MKS S_Gen L v2.0 along with Makerbase TMC2209 v2.0

Link: https://www.youtube.com/watch?v=tVlYURu9AsQ&t=7s

My questions are:

1. What is the maximum allowed microstep rate while slewing and idle? I need the maximum microstep rate for enhanced accuracy (supposedly).
2. Why Onstep recommends 64 microsteps?
3. Can I achieve 256 microsteps or128 at least?

(Apologies if my concept of microstepping is wrong)

Fleep,
For starters, use Stepperonline's precision planetary (PLE series).  You buy the planetary and the motor separately.  It will run you $65-70 for the pair (vs$59 for above).  The precision planetary has much better backlash specs and is a heftier design.  You also have the benefit of selecting different, higher torque motors (e.g. Oriental motors).

https://www.omc-stepperonline.com/ple-series-planetary-gearbox?mfp=300-suit-for-motor[Nema%2017]

With that said, you are still looking at 20arcmin of backlash for the 100:1 planetary.  Since your design has no further reduction, that translates directly to the telescope pointing and guiding capability.  The images provided on Roman Hujer's website are widefield, so keep your expectations realistic.

Regarding micro-stepping, high torque makes the individual micro-steps not equal, meaning it is not a linear function between full steps, but this has not hindered most applications.

		Final Stage	Pulley or gear	Motor Gear set, Planetary, spur, etc	Micro Steps	# of Steps	Pre-worm  reduction  ratio	Single Axis Steps/deg	Tracking resolution, arcsec
Case 1	RA Axis Slew	1	1	100	2	400	100	222.2222	16.20
	DEC Axis Slew	1	1	100	2	400	100	222.2222	16.20
	RA Axis guide	1	1	100	64	400	100	7111.111	0.51
	Dec Axis guide	1	1	100	64	400	100	7111.111	0.51

As you can see from above, at 64 micro-steps, you get about 0.51arcsec resolution per micro-step.  I don't think you need more than that for this level of precision.  If you use different micro-steps for guide and slew, you can keep the resolution for guiding and still get decent slew rates.  256 micro-steps will still be below the recommended 62K maximum (~@28K).

Bob

My 10:1 PLE series planetary.