If a kart is setup properly, it should be allllllmost on the brink of hopping, because that means you’re getting maximum usable grip.
I would suggest tightening up the 3rd bearing and seeing if that fixes it. It’s a small change but it’s best to try something small in that direction before going gung-ho and changing an axle. The axle might be too much. From personal experience, it’s very easy to make too big of a change, gain nothing in lap time, and end up on the opposite side of the issues you were trying to fix.
It sounds like you needed more front grip, so you added the gold bar, which then made the rear flex too much on entry, resulting in a hop. Widening the rear by 5-10mm would probably help too, but 54 3/4″ is pretty baseline so I would just leave it for now. Maybe if the 3rd bearing helps but doesn’t quite fix it fully, you could widen the rear out just a bit.