Hi Jeroen + Marcia,
iS145 has some advantages that could be useful in this situation. Apart from the size is well suited to a "safe" combination to use together with a larger rider on FW equipment, the extra option of 3 strap/stance positions allows for more accessible stances to be used (than the more locked outboard "race stance" of the iS133).
The iS133 obviously requiers a little more wind/skill/technique/determination to get it planing in lighter winds, and ultimately it wont deliver the same extreme upwind/downwind angles of the FW in "typical" FW conditions, but it certainly will do better in lightwind threshold than your current C133. iS133 also has the advantage of being an easier board for lighter riders to ride at medium to hi speeds (once fully planing, c/w iS145) - and of course, iS133 fits much more nicely between the F160 and iS115 (just in case you were ever tempted to try and have a go on it
And with 60kg and RS6-8.4m, it will have plenty of light wind potential.
iS145 is probably the "easiest" intermediate choice - but iS133 might be the more rewarding one all round in the longer term!
Definitely worth a demo/test if you can ! Have fun !
Cheers ~ Ian