OK, it was time for this.
Took the trigger part from my slingshot pistol and eliminated the two problems I had with it:
1. The low power (I simply gave it 87 cm draw length)
2. The dangerous and awkward loading (I added a sliding fork, so you can load it easily and cock it when you are ready to fire).
This is technically and legally a crossbow now, so you have to be 18 to own it in Germany (no problem here - sigh...).
It is a great shooter! I am amazed by its accuracy. I may add a good sighting system.