I’d consider putting a trolling plate on the big 4 stroke and save the 2 grand. I run a honda 130 hp on an 18′ crestliner sportfish and I get 1.8-2.1 mph on a low end via GPS. (not apples to apples but something to consider anyway) If I want slower for running spinners I use the bow mount. 24v is the way to go there if you ask me.
Now going out on Lake Michigan there is always that comfort factor of having a 2nd gas motor on the boat. I know a few guys that would have had a hard time on the big pond as waves were picking up if they didn’t have that kicker motor when the big one decided it wasn’t going to start. If I spent more time on the big pond going out 5-10+ miles I’d purchase the kicker just to be sure I had a decent backup plan in case of trouble.