@Miquelin Those instructions only work if the Nano has a working boot loader. If it is corrupt or missing you won't be able to upoad via USB, it requires an AVR programmer (or an Arduino devices configured as such).
@hami Correct. 1/16th microstepping is going to be smoothest but also the lowest torque. Full mode would have maximum torque but is very rough stepping. Resolution and feed rates are also considerations but this also varies depending on the drive mechanism (e.g. ballscrews improve performance but cost more and may need higher torque).
You can use a unipolar motors as a bipolar, just use the 2 coils (outside taps), this configuration provides increased torque but since your axis has such a fine leadscrew that is likely not required so they went with the simpler unipolar configuration.
I would recommend using the standard drivers and bipolar connection.
@alkassab This thread is in the EleksMill section and discussion about Eleksmaker A3 Pro laser so I'm not really sure what machine you have. Elekscam is obsolete software and only works with the old Grbl 0.9, but if you have a servo then Grbl 0.9 would be correct as it can easily switch from servo to laser.
I'm not sure what this has to do with EleksMill, but yes it's possible.
T2Laser outputs G-code (Grbl based is recommended) so any controller that supports this standard will work. The Chinese K40 machines use proprietary cards but you can swap them with an Eleks or other controller that supports G-code.