My laser is now working as expected!
I’m in the process of engraving faces in plywood which works just fine! I laid three cables from the SKR V1.3:
GND board to GND PSU
Fire Board to L PSU
PWM board to IN PSU
By the way, you don’t need a conversion from 3.3Board to 5V PSU because you shouldn’t get 100% power from the tube anyway.
If I put 60% power in Lightburn then I have about 12mA on the current display. Simply connecting the PWM board to the L PSU is basically possible, but you still have to manually set the respective good maximum current on the potentiometer. The setting via PWM is then not very finely feasible compared to wiring now with both connections L and IN to the board.
Also PWM board to IN PSU and GND board to gnd PSU cannot work because the laser does not fire as long as L is on +! In order for the laser to fire, L must be placed on gnd. You could of course connect L PSU to GND, but then the laser is always sharp and you can no longer trigger it with the test button!