I have been several times in Poland, as my wife is Polish.
I was in Leba / Baltic sea and also in the biggest Mazurian Lake, i.e. Sniardwy.
Sniardwy is wonderful for Formula sailing. In summer almost every day 8-12 knots and rather stable. Exactly the conditions that I like for Formula.
I don't know Zalew Zegrzynski but I googled it and it seems to be a rather small lake indeed, so I expect a lot of lulls. Not that good for Formula
I own myself a 8.4 NP Slalom MKII that I would like to swap for a NP RS:Racing or a GUN mega XS, don't know yet. Anyway the Heliums are lighter than the NP Slaloms. The lighter weight combined with a smaller size will certainly help uphauling. But I dont think that the Helium has more power than the NP Slalom, because this latter has really a lot of power, for me too quickly uncontrollable in the upper wind range. If you need to go down in sail size because of your backbone problem, I would not go too small in board size. Perhaps the Ultrasonic147 is something for you. Should also be ideal with your 9.5 NP MKIII.
Myself, I sail a lot on the North sea and the lakes next to it, more in Holland than in Belgium.
It are similar conditions like the Baltic sea (Leba e.g.), i.e. very good in summer, but much too cold in the winter.