So, this is the status at the moment. Things I updated:

 - purchased a joystick to replace the crappy gamepad with

 - electric height adjustment works (added linear potentiometer and software)

 - mounted bumper properly

Video footage in rather embarrassing english:

Sorry for the low quality picture as well as audio (filmed with phone in garage due to darkness and wetness outdoors).

Next things on the ToDo-list:

 - recraft the bumper from aluminium, since this hobby-glass-thingy just isn't the real deal 

 - Continue with battery pack (need to find some spring loaded contacts wich can handle 30 amps)

 - Build proper (less noisy and vibrating) mowing plates (although I yet lack a good idea on this)