I agree with Remi - a fin will make a little difference, but the sail is the real issue.
A 90 kg sailor can easily handle a 10, 11 or 12 meter sail. However, a new sail will require a new boom and mast, all pretty expensive. If you are really interested in early planing (you have an Apollo), then get a sail to complete the package.
I weight 79 kg, am 64 years old and have a sails up to 11.0 for my formula board (8-14 knots is the range for me on this sail). I drop to a 9.2 for 13-18 knots.
If you were to go for only one sail now, you could buy an 11.0 and if you wanted to, you could add a 12.0 later which would fit on the same mast and boom.
Never buy a Ferrari with a 4 cylinder engine.
Formula 160; iSonic 111; HiFly Move 105; Tiga 263; '85 Mistral Superlight.
Maui Sails TR 11.0; 9.2; 8.4; 7.6; 6.6; Maui Sails Switch 6.0; 5.2; Maui Sails Global 4.5; 4.0.