Try and find a two blade to play with. Less blade area means less parasitic drag. You were correct in your thinking on those two propellers.
The "disc area ratio" of a propeller (how much of a circle is occupied by blade) can make a big difference in the way a boat performs. We don’t have many choices in props for these engines but there are some out there. Can always add a bit of cup to the smaller prop as well.
Still think you have speed left in that one.
My 15’6" fiberglass Gheenoe (with the 18) ran a solid 27mph with a trolling motor battery up under the front seat and some water in the live well.
Your boat has to be lighter and should respond to a good prop.
