@Pequenita sadly my Fanatic just stares at me every time I go to my storage closet at the mountain....
I've still been skiing so it hasn't been a priority. Plus some of the closer lakes to me were not even accessible by road until now. I love the Fanatic for easy mountain lakes, light enough to carry and just enough floatation for me. Remember though it's shorter than what you are looking at.... 7'10" x 28 x 4.
I was looking at the Starboards web site and not sure which exact models you were looking at?
And yes, you have identified your challenge in your last sentence
That and IMHO you really need a goldilocks board 8'6" 26" 3" would be perfect dimensions and so either board is going to be a compromise in some way or another. I think part of your dilemma is that you don't need the volume of board to float you so the narrower board looks more appealing. But I wonder what the exact volume in Liters are for each board?
Have you been able to demo either board?
Are you going to focus on racing? or Are you taking this to multiple lakes, rivers in variable conditions?
If you are racing you would probably want to be on a longer board if you were seriously competitive? The 23 might be a smidge narrow in the more playful conditions you described. Great for distance, not so good for variable water. The benefit of the 28 width is that it's just narrow enough for a smaller person to not have to reach and bend when paddling but adds a bit more stability. The 36" you were on recently must have felt really wide. 23" is what my ocean board is (again just a softech hard bottom 10'6" surfboard) and I'm usually only on it when its flatter and I don't surf it. I like how narrow it is b/c it feels streamlined and I feel a longer narrower board is better in ocean swells for what I am doing (cruising). If I were surfing I would be on a short fat board.
I will say even my one day on Lake Tahoe with the Fanatic I could see the benefit of having more board once the boats started creating wake. Again the Fanatic was more of how small/ light can I go for hiking in the mountains to lakes and still have fun. I haven't been on your river but sounds like there are some interesting rapids and so it seems for playing round I would probably choose the wider shorter board. It would also be the easiest board to re-sell if you didn't like it.
As far as fins, yes large single fin easier for distance. 3 fins (thrusters) easier to maneuver, less power/tracking for distance. But why not have both? Is the center fin removable on the 9'6"?
The Fanatic has 2 small side bite fins permanently attached and a removable center fin (surf fin box) so I am able to put in a longer or shorter fin depending on what I need. Check to see if you can change out the fins on either board?
Can you provide a link for the two boards you were looking at?