If you’ve already got the 15m, then keep it. I’ve had the opportunity to compare my old 14m Rise with the new 12m Razor and although they were close, the 14m had the extra grunt I needed to make the session a good one without a walk of shame. The Flite will be better still, so keep it.
Coming down from that, the 12m is a great kite, and sits nicely in your quiver range. As you already have that too, I’d keep it as well.
Whilst I love getting out on my 10m Razor, it is a little close to the 12m. A 9m would make more sense, but personally, I’ve got the 8m as well, so it fills my gap nicely, but if you’ve the opportunity, I’d sell the 10m and get a 9m Razor.
Then if you’ve the money, you could get a 7m for those nuking days - but that depends on how many you get a year. You should be able to hold down the 9m a fair bit.