There are several things that need to be checked and tested before you split that tractor.
First off, have you changed your transmission filter
A restricted filter will cause exactly the symptoms you have.
Secondly and most importantly is you need to check your clutch operating pressure, not as mentioned previously the lube pressure (which is only 15 PSI). Depending on what the results are after testing the clutch pressure, that will determine how many other tests need to be done.
DO NOT down play or cut corners on the importance of these tests
as this clutch is held engaged by hydraulic pressure, so if the clutch operating piston or the lines leading to it are leaking internally, and you don't conduct the tests properly, you could end up splitting the tractor, installing a new clutch and still have it slipping afterwards because you never fixed the real problem.
After changing the filter and if there is no change in operation, roughly below the left front corner of the cab there is a valve body that has linkage connected to the clutch and PTO levers and several test ports, this is where you will be conducting your pressure tests.
The ports are labeled with names cast into the valve housing. All tests must be done with oil at operating temperature and this is important. First connect a 300 psi gauge to the port labeled CLUTCH. Pressure at 1900 RPM should be 170-180PSI.
Drop engine RPM 300-500 RPM, observe how much pressure drops. Increase RPM to 2200 and observe pressure increase. If in either case the pressure change over the initial pressure at 1900 RPM is more than a few PSI, this is indication of either low oil flow or internal leakage.
If pressure remains constant at the 170-180, then most likely it is in fact the clutch pack itself that has failed. If the pressure is lower than 170 PSI, before you do anything else, you test the operating pressure of the PTO clutch.
These tests aid in determining if it is internal leakage or low oil supply from the charge pump. With the gauge connected to the PTO Clutch port, with PTO engaged at 1900 RPM, pressure should be exactly the same as the initial Transmission Clutch test pressure. Check the pressure at the lower and higher speeds as with the clutch tests. If the PTO clutch pressure is overall always higher, this is an indicator the transmission clutch piston is leaking or the lines running to it.
If after testing both clutches the highest pressure obtained at 1900 RPM is below 170 PSI, it is necessary to adjust the system pressure regulating valve and increase the system pressure to obtain 180+/- at 1900 RPM and conduct the tests again.
I cant take the time to tell you everything about how to do this, this is a rough description of what needs to be done before anything is taken apart. If you have any questions, be as specific as possible and I will try to explain what information you need. Most of this info is contained in the I&T manual for that tractor which is available new for around $30 if you dont want to spend the bucks on the Deere ones. I have both.
You mentioned that hex shaft that drives the charge pump, yes it needs to be removed when splitting and remove it completely, don't leave it sticking out the back of the tractor even a foot or so, I learned this the hard way
Got doing something else and ended up bending one when something fell against it