I think you need to come to accept realistic terms with what you have.
Running large 25mm screws at high speed with these motors and drives isn't going to happen.

You have a 6 volt motor that to accelerate up to speed is expecting to see 6 x 20 volts or around 100 to 120 volts and you only have a 40 volt max supply that is really only going to be happy on about 32 to 35 volts to stay inside a decent safety factor.

Gearing up, then micro stepping will only bleed off available torque making the situation worse.

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