@Patric-Wilms I think your concept is correct.
Most probably that the 5V pins 2 and 28, the 3.3V pins 3 and 30 and the GND pins 1 and 26 and 29 of the header are common / joint respectively on the Relay Expansion. You could check this with a multimeter / ohmmeter easily.
And please correct me if I'm wrong because I don't have such genuine Onion board.

Powering the Omega with No Dock has "the only procedure recommended by Onion" - but the input and output capacitors of LD117 3.3V voltage regulator are missing from that circuit - I don't know why (maybe intentionally ;-).

Good luck.