This is a good example of where a hyd pressure gage installed would help with trouble shooting.
If you raise a load , and turn off the engine, does the bucket instantly dump the load, or is it a slow dump?
FEL valve should lock the load with the levers in neutral whether the engine is on or off. Expect some leak down.
If the gage was installed in the pressure port of the FEL valve, anytime you activated a cyl, you could see the pressure the system is generating. If the relief valves never operate, then the fluid is leaking from the valve or bypassing in the cyl.
You can check each of the cyl to determine the problem.
Does the 3pt lift a full load, and does the relief operate. Some components in the 3pt might be sticking. Might need removal, inspection, and cleaning or replacement.
The knob under the seat just sets the implement drop speed.
Your FEL valve should have a relief valve close to the IN or pressure port.
Picture is just for illustration.
https://www.surpluscenter.com/MorePhotos.asp?ITEM=9-7976
Here is a relief valve on a Prince valve .