I'm having a hard time finding a shop in Europe where this motherboard is available. Have you got some links?
I was also wondering if the motherboard had to be powered by ALL 24 pin connectors or if 3 molex + 1x 24 pin is enough?
You don't need to use the 4-pin molex connectors on the motherboard when using powered risers. They are only there for the few people that may still use ribbon unpowered risers.
https://youtu.be/6e7ywLfRogQ?t=4628