Reducing microstepping won't improve the torque.
Are the drivers getting hot? If so then you may need to cool them with a fan.
Don't assume that the voltage settings you used on the A4988 drivers give you the same current on your new drivers. It depends on the sense resistors on both types of driver. By my calculation, if your TMC2100 driver has 0.1 ohm sense resistors then 1V will give a current of 1.28A.
Are the drivers getting hot? If so then you may need to cool them with a fan.
Don't assume that the voltage settings you used on the A4988 drivers give you the same current on your new drivers. It depends on the sense resistors on both types of driver. By my calculation, if your TMC2100 driver has 0.1 ohm sense resistors then 1V will give a current of 1.28A.