The four main motor speed limits in a 3d printer are:
Firmware step generation frequency limits
Firmware motion planner effects
Loss of torque and precision due to motor coil inductance and back-EMF effects
Step generation rate limits will depend on the firmware and controller board used. There is a significant range, particularly when ...
Focussing on the questions at hand:
Is it possible to use full steps?
This depends on your printer board. Many boards use dip switches to select the (micro) stepping mode of the stepper driver. E.g. a RUMBA board has dip switches located underneath the stepper driver boards (e.g. DRV8825 or A4988). Sometimes you also see jumper caps. The answer is both ...
To answer each point:
Microstepping improves resolution as much as 32 fold (I think) but reduces torque the higher you micro-step.
Torque is not reduced by micro-stepping. Torque reduction only occurs when you are moving at high RPMs. The motor's phase resistance has to be conducive to the target RPM's (or step rate). Further, micro-stepping can go as has ...
You will likely not see a noticeable improvement by upgrading from a 16x or 32x to a 64x or 128x microstepping driver. Depending on the motors you're driving and the size of the load you could actually see a decrease in quality.
Although microstepping increases theoretical resolution it does not necessarily increase accuracy. The reason is that ...
Prints benefit from higher microstepping in two ways:
Using microstepping reduces noise from your printer's operation.
Using higher resolution microstepping does not increase the physical accuracy of your prints meaningfully, but it can reduce surface artifacts such as moire.
There are extruders that use flex shaft to turn worm gear in direct extruder while motor is mounted on frame which turns flex shaft (zesty nimble comes to mind). Why don't they just use smallest possible stepper motor to rotate worm gear directly, instead?
The Flex3Drive, from which the Zesty was purportedly cloned, does admit using a very small NEMA-8 ...
There's an old rule that says "If the question starts with 'Why don't they,' the answer is most likely 'money.'"
In this case, the issue is the cost of worm gears. Properly mating worm gears are much more expensive to make than common spur gears. That probably accounts for most of it -- not to mention the 200 step motors we see on most FDM ...
I don't have these controllers, but I read that with default settings the TMC2208 will interpolate the microsteps set by the I/O configuration pins to
256 microsteps. Please look into how you set up the dip switches / jumper caps on your board, it seems that only 2 are used (MS1 and MS2). Furthermore, can't you just decrease the count of the array ...