Which stepper motor should be enough to move a water valve and for the TMC2130 driver?
This is not how you make a simple hydraulic linear fluid valve, but one that is precision controlled to your specs.



In order to make it linear, you need to know the torque transfer function might not be position-dependent with ageing on the valve or water pressure.  But you do need to control torque with a motor. The best way is to use current to control the motor torque limit, but you need to define position, velocity and acceleration as ther emay be some backlash.  You can attempt to measure it and then do the same for your stepper motors vs voltage or current limit then define a,v,x transfer functions.  To allow cogging with low torque limit, recal is needed to home position.  Full Steps can be used.