Please don't surf behind that outdrive. For everything else it can be great.
I would suggest going with a vented four blade prop and finding a prop shop that will let you buy one and try others to make sure you get the best one for your application. Dropping the pitch to 15" will make everything better for your purpose but maybe one prop model or another will be better. Also, maybe a different pitch would be best. I don't know if a five blade would help but, at that pitch, I think a four blade would give the best results.
When using the sterndrive for water sports. it will often be best to keep it trimmed down on starts and then trimming up some, perhaps 25%, to load the stern of the boat. Have your people ballast in the bow during the start and when running, as much as practical. Some type of foil on the leg can help maintain speed better below planing speeds. Tabs can help but they can make the wake less clean for boarding, on some hulls.
Another possibility is to use the PerfectPass in RPM mode. It will do less searching and not overspeed like it does in speed mode.
Boarding and Airchair really need a consistent speed to enjoy so I hope you get it sorted. It won't be as easy as that Nautique because of the drive and hull design but it can become very good.