Yes, well it's all about compromising isn't it.
Look at my 4 board slalom quiver.
I'm very happy with it because it fulfills perfectly all my needs without any compromise. I use on each board maximum 2 sails and each sail also overlapps 2 boards.
Nevertheless, from a practical and economical point of view I'm also thinking of reducing it to a 3 board solution, i.e. replacing the IS122 & IS101 by the IS107. Yet, I'm not convinced because I feel I will have to sacrifice something in certain wind ranges.
Personally I think that if you want to do slalom in conditions from ultra light wind to heavy wind (8-30 knots) in a rather comfortable and decent way, the bare minimum of boards is 3.
Your 2-board solution with the FU93 and FU133 should work quite well, but like you already point out yourself, you also feel you're missing out on something in the ultra light conditions, especially on smaller lakes where the wind is not always that stable.
No one can tell you what's right or wrong. It's something that you have to decide for yourself.
IS87 / IS101 / IS122 / US147