You may be completely aware of the concepts below, but to cover the bases, I decided to throw this in the mix. None of the replies specifically cover that the HST pedal is not an accelerator, rather is linked to a hydraulic pump which feeds a hydraulic motor. You effectively have 3 "ranges" with an infinite "gear range" within each range if you use the HST pedal correctly. The harder you push the pedal the more VOLUME of hydraulic fluid you push, which equates to a higher "gear." As you hit an incline, at the point that you start losing engine rpm, release the HST pedal to increase the PRESSURE and decrease volume. The rpm will come up, the tractor speed will probably drop, but you should keep moving. This is no different than had you changed to a lower gear with a gear transmission. Many people expect to maintain tractor speed on an incline in high range, but they most often expect too much. Resist the urge to "give it more gas" with the HST pedal. That is counterproductive for reasons explained above.
I have a lower horsepower Kubota than your tractor, with a backhoe no less, and I can climb a fairly steep hill in high range with a bucket of dirt as long as I heel back as necessary to keep the rpm up. I keep the engine loaded to keep the governor open, but don't let it pull down. The tractor slows down but I do not have to change ranges, which overall saves time. I do this at about 2200 rpm, so I am not running full rpm and I get a lot of work done by fine tuning HST volume versus pressure.
I hope this helps. HST use is the same, regardless of brand, so this is applicable to almost anything except the Kubota HST-Plus type setups. They were designed to make it easier for those who have difficulty making the transition from automobile automatic transmissions to an hst, which ISN'T an automatic transmission, and requires a different operating technique. I have an almost 40 year old Wheel Horse HST that I successfully operate the same way as I do my
B7510.
You will eventually find what works for you.