I ride footstraps which do tend to come out, specially when I unintentionally launch when trying slide transitions

This is something that I noticed with the Airush boards, I could never get the straps sorted on the demo's that I tried, the boards themselves are great but the standard pads and straps could be much better (IMHO). Get your screwdriver out and play with the footstraps, get the dang things to adjust down tight enough so as your feet do not rattle around.
You need tight fitting yet comfortable footing, especially on a big board. Big boards tend to slap the water harder and they will skip in the chop when you start to get overpowered, if your feet aren't secure you will lose your edge too easily and end up on your arse. A sharper rail is going to be a little less forgiving in the rough. It's not just a matter of applying more back foot, you also have to use the edge and the fins in unison. The way you ride a big board (when powered-overpowered) will vary along the board as the rocker and tail width varies. Just play around with a few speed runs when you have the room for wipe-outs.
The Airush Flow (and other SLE's like the Waroo etc) are particularly adaptable at being ridden powered or underpowered, something that doesn't just depend on wind strength, you will learn how to do both with practice. Your particular kite and board combination is going to give you a huge usable wind range.
Personally, given your wakeboarding history, your instructor was correct. The 138 would have been a more enduring choice. On the upside , something to consider for next season maybe, a two board quiver is good and the purchase of a 130-133 would see you set for just about everything short of the purist surfboard experience.
Just for the comparison, our weight is similar. I ride a 125, a 138, and a 167 without fins (for when it just isn't blowing hard enough yet). Kites are 9, 13, 17, and 20... yeah yeah, I need a garage sale.
