Got it, thank you for explaining it better.
I was looking for possible explanations to my burnt mega, and thought that the floating and/or the missing 5v load was the issue.
But… I grew some balls today. Connected the ramps to the notebook with the 12v PSU and it is working!
I did use an old ATX power supply instead of the switching one, so I still don’t know if the problem was due to the PSU or just a faulty Arduino Mega.
Thanks you both again for your feedback.
Now it’s time to cut the threaded rod and finish the Z assembly.