The optimal solution might be a different boat or a different engine
I wonder how big a factor the engine RPM/longevity issue is. Are you
using an outboard or an inboard? Gasoline or diesel?

One option could be to get a propeller with lower pitch. You could run
at optimum RPM but lower speed. Such a propeller might double for
trolling purposes as well, where you need lower speeds. Around here
people into trolling often get a 2nd outboard, and at least sometimes a
2nd (or 1st :-) propeller for trolling instead.

Risto