Having windsurfed actively in the Helsinki region for many years I think I should pitch in.
An iSonic 133 with a 10 m2 sail will for sure give you lots of TOW during the warmer months of the year. The downside is that there are very few 85 cm wide slalom boards in the area, you will be pretty much alone with this style of equipment.
If you would go with Formula you would for sure have more company. There are typically a number of Formula sailors out whenever there is some wind. As you seem to be an experienced Formula sailor on the heavier side I think as 12 m and 10 m sail would cover pretty much all conditions (until you can jump on your 100 l freeride). Maybe even just a single 11 m2 sail would work, if you have a 6.5 – 7.0 sail on your old Freeride.
I am myself about 10 kg heavier than you and my big board is a 74 cm wide freerace board and a 9 m2 Sailworks Retro (next one down is 106 liters and a 7m2 sail). This combo gives quite a number of days during the season, but does of course not get going as early as a Formula with a big sail.