Echoing some of the suggestions here with regards to having too much caster and possibly going to higher rear pressures. Sounds like you’re already at max rear width, otherwise that would be my first shot.
Basically what’s happening is you are loading up the chassis too much at max load, like Dom said. That means too much weight transfer is happening. To reduce weight transfer, I would start by reducing caster, as caster can have the double-benefit of also reducing front scrub and helping the kart roll off the corner better.
I would also take a look at either lowering the seat or lowering rear ride height to get your center of gravity lower. The ride height is something I play with pretty often on the OTK, though I’m usually raising it because of my size.
The H axle is the right direction. I don’t find much difference between the H and the HD or HH. Those are worth a shot if you want to go stiffer, but I don’t think it will make as big of a difference.
The front bar vertical is a mistake in my opinion. You’re correct in your thinking that you are stiffening the kart and perhaps limiting flex in the rear, but you’re also adding a bunch of front weight-jacking into the kart. Always try and picture the chassis as a spring. When you have a hop, you are overloading that spring, and it’s snapping back and oscillating, causing it to load up, release, load up, release, repeat. When you stiffen the front bar, what you’re really doing is stiffening the “spring” of the frame and then, with the additional weight-jacking, increasing the force with with you load that spring, so it could actually make the kart hop worse if you stiffen the front bar.
Reduce caster would be my first shot. You can go negative on the top pill and keep the bottom the neutral pill, but I probably wouldn’t go full negative, with an eccentric in the bottom too. If you go to the half-negative and it’s still a little hoppy, I would tweak something else too to fine tune it. I don’t like to max out one adjustment in search of a fix if I can avoid it, because once you get to an extreme like that, the possible side effects of that adjustment can start to rear their heads.