@licantrop Just play close attention to the wiring - double check everything before you turn on. You could try even lower motor currents just to test. Then if something goes badly wrong, a motor will stall before any physical damage ensues. If you set the current too low, you'll get missed steps or the motor might not turn at all but you won't damage anything.
Once you know that you have basic movement, you next step should be to fit the end stops and/or Z probe. Make sure the end stops work first, then do the homing. Once you can home all the axes to their minima, then jog them all to their maxima and note those distances. Put those values in config.g (M208 S0). The configuration tool can only work on the values you've put in and the true maxima may be less than the 300x300x300 you've set. Once you can home the machine from then on, any moves you attempt will respect the axes limits and won't go beyond them. So you can remove the M564 H0 if you used it.