I've used IB (and don't use robinhood). The IB GUI is a fucking trainwreck. If design had ethical obligations the way bridge engineering does, IB's designers would be in prison. It is bad. And it's still one of the better trading interfaces. Ever seen a bloomberg terminal?
That jump from 2.4 to 6% is effectively a "regular people can make sense of this without wanting to kill themselves" markup. No different from the markup any other SaaS company puts on "run this 10 line script periodically". One can sell convenience.