In general, Formula boards, when used for racing, need sails with a lot of loose leech to cover a huge wind range to go very deep down wind as well as reaching and upwind.
If you're using Formula boards just to recreationally sail light winds (like I also do), you might say that a freerace sail is sufficient. But unfortunately I found it is not the case. I have a 11m2 pure race sail (Gun Mega XR) that I can hold just as long as a 9.5m2 freerace sail with much more control and power at the same time. The 9.5m2 is a light sail with very light handling that gets me going with only a few knots more than the 11m2 but then it very quickly overpowers me when the wind picks up and tends to launch my formula board into the air.
So if you're lucky with the wind range on a particular day and it fits exactly in the rather small wind range of your light freerace sail, you're OK. But I'm afraid that most of the days it doesn't and you should definitely go for a pure race sail, meant for formula.
Regarding your backbone problems, I understand that you want to go for a smaller and lighter sail.
The pure race sails are heavier and more difficult to uphaul, even with the right technique, that's for sure.
I also avoid to let my rig fall into the water. There are formula days that I don't need to uphaul at all, because I really pay attention
PS: Where are you sailing ? Your name is written in a Polish way, so I was thinking of Mazury or the Baltic sea ?