I can get pressure gauges
A good start. You're going to need three pressure gauges (preferably identical if you can get them) with a maximum reading somewhere in the 5-600psi range.
We're gonna have to invent a testing procedure because the manual calls for all machine systems to be at normal operating temperature before testing pressures. Unfortunately that's when your machine goes t1ts-up so we'll have to do it starting from cold and this means the pressure specs quoted in the manual are not going to be all that much use.
Because the first thing you lose is speeds 4 thru 6 in reverse that points the finger at the High Range #8 clutch. The same clutch is also used for speeds 5 thru 8 in the forward direction.
Question: When 4 thru 6 in REV start to act up does the same thing happen with 5 thru 8 in FWD.?
What I'm going to suggest is to put pressure gauges on the test points for both the Low and the High Range clutches and run the test lines from the transmission to the cab. That way you can observe the pressure all the time from the first cold startup to the point where the transmission starts acting up. I'm not too concerned about how they compare to the test specifications at this stage, simply to know how they behave as the transmission warms up from cold.
See the attachment. Hook up a couple of gauges to Test Points D (Clutch #8 - High Range) & H (Clutch #7 - Low Range) and carefully observe what happens both to the maximum registered pressure, and how fast the pressure rises when the clutch engages, when the machine is cold. Check if either the max pressure or the rate of pressure rise changes as the transmission warms up.
Clutch #7 (Low Range) will be pressurized in speeds 1 thru 4 in FWD and 1 thru 3 in REV.
Clutch #8 (High Range) will be pressurized in speeds 5 thru 8 in FWD and 4 thru 6 in REV.