I have a 530 CK hoe with the same issue. I have removed the pedal and torque converter relief valves and cleaned them (even shimmed the spring with no luck). Mine has better pressure in cold weather (I suppose the fluid is thicker then). The previous owner had even taken out the thermostat to try to keep the pressure up. I have a repair manual and will be tearing mine down maybe this summer. There is a set of steel rings on a piston that enguages a clutch pack and I assume these are the culprit.
I have been using mine carefully as is and downshifting when I feel it slipping. I switched up to 50% 90 weight john deere hydraulic fluid and that has helped my pressure somewhat. I also bypassed the radiator caseomatic fluid cooler so that the hot engine doesn't warm the fluid up (I never drive it over 200 yards at a time). I can scoop a loader buscket full of dirt, if the fluid is cold in 1st low range but can't drive in high gear either.
The steel sealing rings are about $90 but the clutch pack parts are several hundred $