AntoineS said..
Hey Dave, Martin Brown and Patrick Spiers on the Kosi Kiters group will be able to answer with more details, but I'd say the snow at the top is usually good from mid July till mid/late September.
I wouldn't buy a snow specific foil kite unless you're going to spend days and days up there, you can already have a lot of fun with your current water kites. That said they do make life easier with no pump to carry and ease to selflaunch and land. As you've read the wind is tricky, most of the time either nuking or non existent. And always very gusty because of the geography obviously. So if you've got one a wave kite does wonder in the snow as they're very stable and able to drift when you go downhill and they usually have a lot of depower.
I think your best bet for a reliable forecast is crossing data from the BOM and Willyweather. From memory the most common direction is south-west.
Hope this helps!
Can't wait for the snow to arrive, see you up there!
Mate i was going to take my 14.5 vegas you rode the other day on full wakestyle settings

. The wind rose for thredbo village says south west is the most common.