You are under-propped and hitting the rev limiter. Our similar rig worked well with the basic Yamie 14p 3-blade Al. However even better for most of our use is the 13p Nemesis pontoon prop 4-blade (lower cost version of the Spitfire pontoon). For pulling an inflatable with 4-5 aboard, the 12p Talon 3-blade is the best I've used, but you may not want to go that far. Your rig may hit the rev limiter with that one. Our relatively heavy R-model almost does.2023 Model 20LSNP
Dealer originally gave me an aluminum K prop 14-11. WOT exceeding 6200 and cutting out
Does anybody have the same setup. If so, what prop are you running?