Don’t change all the variables at once… leave in the 3.75" sides and put in that 6.5" center. All the way back in the box, if you’re worried it will be too squirrelly, but I think you’ll like the setup fine without the larger sides. I run 3.75"'s and a 6.5" in an 18x23x14.25 9’10" EPS/epoxy and its great like that…
I think you’d like a twin keel fish much better if you got it right. The wheelbase on a 9’ longboard is pretty long if youre trying to ride it from the center.