First thing to do on a motor that has sat unused for a couple years is to change the water pump. Impellers lose their flex over time, especially if left dry. Very likely that is the case with your motor.

So, if you havent already, do a FULL water pump kit, including the housing.

Then test water pressure again, preferably with the lower unit submerged in a tub of water, covring the anti-vent plate by a couple inches.

Do not attempt to run the engine with the hose hooked up to the flushing port!! They are not designed to use with the engine running. Ever. They will however run water thru the upper engine very effectively and can help flush dirt etc from the pisser tube.

If you use muffs, the small round ones work best with the Suzuki's, the larger square or rectangular ones just dont suit the shape of the lower very well and dont give enough water to the pump.

Mostly obstructions in the pisser tube can be easily dislodged by running a foot long length of heavy nylon fishing line or weedeater line up there while the engine is running, the water pressure helps push out any dirt and grit that is lodged up there.

That small nipple is only held in by two small prongs or plastic hooks, and sealed by an O ring. Just gently pry it out