Wondering if a starboard 2019 isonic 91 is a suitable super light wind board as the dimensions are nearly the same as a jp slw. 7cm shorter and 1cm less in width, or are these for racing and harder to ride than the Fanatic falcon LW and jp slw?

IMO the ISonic is a lot more technical to ride, SLW is like a big freerider.

Exactly. Isonic meant to be ridden with deep angles overpowered while the slw can be sailed underpowered to powered without issues.

That said I used a formula board (old school) for recreational purposes and all I had to do was use a shorter fin.

I went from a Starboard UltraSonic147 to a Patrik Slalom 150 then140 slalom. The 150 was awesome to ride while I've not used the 140 much. The US was great too. We have 4 US147s at our local and me on Patrik.