RE: To Roger Choice of light wind board
Not Roger but if you really want to be planing in true 6 knots of wind, I guess the ultimate board for that is the Apollo coupled with an 11 sqm sail and its soft 75 cm fin. Can't say it will still be fun in 15 knots of wind with say an 8.x sail for your 58 kg, however...
I've tested iSonic 133 and with my 65 kg, I can be planing as low as 7 knots of constant wind with NP RX2 10.6 sail and a 54 cm fin. At the other extreme in 20 knots with a NP RS1 6.6 sail and its stock 40 cm fin, it's becoming a bit of a challenge to keep control and jibe in steep chop owing to its hard and tick rails for my light weight (no problem with a thinner and more soft-railed HS105 of ± similar width). So, for your 58 kg, I have no doubt it'll be much more efficient and enjoyable in true 7-15 knots of wind than your old '96 long and narrow Mistral Malibu.