RE: Trading a Carve 122 on a ISonic 135
My guess would be there would be a significant "overlap" between your FE 160 and the Isonic 135, but you are a very big guy at 102 Kg. (225 lbs.) and the extra volume in the Isonic 135 vs the Carve 122 might be very helpful.
As far as the Isonics being "technical" to sail, I've not found that to be true.
To get the absolute max. speed from an Isonic, yes, you need to be quite technical about your rigging and tuning, but to simply sail as fast as you can, the Isonics have been pretty easy, for me at least with significantly more experience than you've achieved in your 2 years of windsurfing.
I put quite a few demo sailors on the Isonic 115 last year, and most came back raving about how easy it was to go seemingly faster than they had achieved before.
As far as your getting downwinded on your boards, while trying to waterstart, what prevents you from uphauling?
I can waterstart, no problem, but I most often uphaul as it takes alot less time and energy.
As far as staying upwind while you are sailing, perhaps a larger and more vertical planform fin would help a great deal.
Also, with only 2 years of experience you may not have discovered how to sail "on the fin" to get your formula expereince board to go upwind when fully planing.
You can do the same on your Carve 122, but you probably need a larger fin and you need to be going pretty fast to get upwind "on the fin" on the smaller board.
So, the wider Isonic 135 can easily handle a larger fin, will plane significantly earlier than the Carve 122.
This could be a very good move for your overall progress.
Hope this helps,