If you want advice from a pontoon owner who has re-powered and operated a pontoon since 2017, here's this: FYI, FWIW:
It's impossible to tell if a motor is too high or low just based on what you posted. What you posted tells me you are over-propped and lugging your motor.
I do not prop my pontoon for speed, I prop for the higher recommended manufacturer's operating rpm speed....but it's your boat, your money.
A picture of the ventilation plate in reference to the middle toon would help. It should be even with the bottom of the toon. A picture of that plate while running would help. It should be right at or touching the water...If it's below the surface while at speed,then it might benefit to raise it (maybe) ...All of this is a starting reference point. Adjustments are based on mainly if ventilating or not. Pontoons are more susceptible to ventilation due to design. Ideally, you want the motor as high as can be without ventilating in turns or when trimming up at speed. That's the test. I had to lower my new motor to the last position to eliminate that problem. The 11/2" made a big difference, thankfully.....
So, yep, make sure the height is correct as best you can.
All that being said, as stated before by dingbat, you are overpropped. Going to a 3-15 1/4x13 would be a good starting point. Understand that changing diameter, #of blades, different manufacturer, different metal, compared to the original will also affect rpm outcome.
propping can be expensive....
and read this.
