You have done the basic maintenance stuff so we can at least skip that.
Some of the early 254 Jinma units had a separate tank and pump for the steering, some used the 3 point hydraulic system to power the steering. Since you mentioned the diverter valve, you likely have the combined system. Just checking to make sure.
Do you have a front end loader? If so, does it work properly? Have you tried by-passing the loader by connecting the two tractor side hoses to each other and leaving the loader side hoses disconnected?
You seem to have it narrowed down to the cylinder on the axle or the "steering sector" box at the bottom of the steering column. Based on your description, my first instinct is the sector box, but that does not rule out the cylinder.
Here is the next test that I would do. Jack up the front axle so both wheels are off the ground. If you have a front end loader it should be able to lift the front end high enough for both tires to clear.
Then with the engine off, turn the steering wheel. You should be able to turn it in either direction continuously with no hard stops. The cylinder will turn the wheels until they hit the physical stop on the axle, but the steering wheel should still turn past that stop. The steering should feel smooth in both directions but you might feel a little more resistance after the wheels have hit the physical stop.
Then start the engine and test again. It should work the same way, but require less force. If you feel any "catches" or areas of varying resistance, try to take notice of the position of the cylinder and the steering wheel. Keep going in both directions past the physical stop. See if there is any correlation between where the "catches" are and cylinder and steering wheel. If these spots are repeatable, that will give you a clue as if it is the sector or the cylinder.
Let us know what you find out.