Thank you for the quick responses.
In flat water at slack tide with calm winds I can go 6.0 knots SOG at 3000 RPM. .....
How fast your boat is moving over the ground is meaningless.
Start with the RPMs. You said the motor is rated to max at 3600 and it should get there at wide open throttle, when underway. No matter how fast you're going. How fast you're going can be affected by many other things, but getting to max RPM is a sign of properly drive train configuration and engine health. Improper prop pitch can do harm to a motor over the long term.
While low RPM at WOT is a sign of an over pitched prop (ie there is too much drag on the blades which bog the motor down), the tachnometers on boats can be wildly wrong too. I would get a laser tachometer and confirm those settings on the engine itself. They cost under $20 on Amazon, you just put some reflective tape on a pulley and point the laser at it. Once you know the RPM is correct, then mess with the prop. +/- 100 RPM isn't too bad.
I would also highly recommend calling the manufucturer of MaxProp, PYI. They are extremely helpful and I would want to confirm you have the proper size prop in the first place. If the blade diameter is wrong, fixing it with pitch isn't the right solution, even if it's better for the engine. Who knows if the prior owner fitted it properly, or just bought a used one.
SOG may be a mistatement on your part, but it refers to speed over ground, which is measured by a GPS as your speed over the earth's surface. It's affected by current. Hull speed is measured by speed through the water. You may be making hull speed through the water, even though facing an oncoming current and seeing slower SOG. Speed isn't the issue here. Good luck troubleshooting the RPM.