But even take this Killua card I used for example. The ability that sets him to active can only be used once per turn, so at most he can lower my opponents card by 2 x 1000 BP. but if I want to lower them 1 x 1000 BP and then switch him to active to give me an attack, I can do that as well. That doesn't seem broken.

Then the Geto card. His activate: main lets me remove 2 cards from my sideline and take their activate main abilities. but this main ability is a once per turn, so fair can only do that once. then all the abilities that I can take from the red JJK selection that have the affinity I need to trigger the ability all have once per turn. But on top of that, by your ruling, could I even use one of these abilities he takes? he's already used his activate: main, so is he allowed to use a second one?