We run both our tractors in high range.
The b2920 required a lot more foot pressure on the peddle and eventually quit moving.
(right leg gets tired from pushing hard for hours on end)
Do not press harder on the pedal, that is the exact opposite of what you should be doing. That raises the gear ratio even higher and makes the problem even worse.
It sounds like you think the 7610 is working correctly, but I bet the HST is shrieking loudly on it, which means you're in too high of a range or pressing too far on the pedal. In the same situation, the 2920 is simply opening a relief valve to prevent overheating of the fluid and damage.
High gear is for transporting the tractor at high speed. Low resistance ground engaging work and pulling can be done in medium. Low gear is for intense ground engaging work and pulling. The user manual should mention this. High gear is not intended for ground engaging work or pulling, it's really just for transport.
I think you're simply using the HST wrong, both in the range selected, and the fact that you're pressing the pedal down more when you should be backing off. The 7610 is letting you get away with it, but I bet the HST is shrieking loudly and you're borderline cooking the fluid and asking for trouble. The 2920 is going into relief/bypass to prevent problems. Anytime the HST shrieks, it means you need to back off on the pedal or lower the gear range.
I'd almost suggest you should have a gear tractor, but you'd likely find that a high range fixed gear would not have enough grunt and you'd need to downshift to a lower gear. Which is exactly what the 2920's HST is trying to tell you. A gear tractor would begin to bog down or stall the engine when in too high of a gear. But on an HST, which is just a pump, you'll either cook the fluid or trigger a bypass relief. The fluid cannot really transfer the load between the wheels and the engine in a way that would cause a stall -- it will shear/slip first. (similar to a torque converter on an automatic transmission).