I am not sure if this will help you or not but your issue sounds like one I had with my F115 of my 2010 Bennington. Engine would run fine at higher speeds but would stall at lower speeds especially when docking, shifting from forward to neutral to reverse etc. Like you changed everything with regular maintenance but was still having the issue. This went on for 5 or 6 years with multiple techs looking at it. Like a car many times it wouldn’t do it when the techs would take the boat out. Finally a different set of eyes was working on I explained to him what was going on. As I was standing there talking to him I just happen to look at the gas line as I have had them replaced a few times before because they get old and start to crack and leak. As I did that he just happen to notice that the gas line just inside the cowling was light gray in color. He felt it and said that line there is an original gas line and said I bet that is the problem as the lines break down and start to collapse restricting the gas flow causing the engine to stall at the lower speeds. He changed that line and the boat never stalled once last season. I have since gotten a new Bennington this year. Just something for you to take a look at.
