Update, for those interested:

Switched everything out with a mechanic, spent a couple of hours on it. Eliminated all components one by one, including the wheel untill the whole drivetrain was changed apart from the BB, which has already been replaced and it still does it. He got it to partly present on the stand in the big ring, said it was bad and seemed to be coming from the rear.

Eliminated shoes and cleats by transfering to old bike and rode that, which was somewhat of a relief tbh.

The only assumption left is that it's some kind of frame defect/misalignment. Being returned to manufacturer.