Hi guys,
The new board arrived, got flashed, got tested, build in and works perfectly.
The drivers just slided in with no resistance at all. Couldn’t get them in completely on the last board no matter what.
Also the TFT scrambling is much better. Not completely gone but huge improvement.
Many many thanks to everyone involved in the solution finding process for my initial problem! It was a delight to work with you and I learned a lot 
When comparing the old and new board, I noticed, that two chips (?) have different positions; the ones next to the jumper that defines power input. Seems to play no role?
Edit: I have one question that might be connected to the previous issue. When the stepper motors moved once and are then in resting mode, they buzz. I read that “low quality drivers” can be the reason for that. All my motors do the buzz. Could it be, that my driver’s were compromised when the board got fried? Or is the buzz unavoidable?
I still have some questions but these will need another thread if I can’t find the answers anyways.
Cheers,
Karl