If you didn't have an 11m already and had to buy only one kite it would be a 9m but a 9 will overlap too much with your 11m.

You only get out on a 7m around 20-30% of summer but if you can afford a cheap-ish 2nd hand 7m as well keep it handy for the nuking days and also a bit of winter kiting if you are up for it.

If you can get both 7m and 9m - go for it. Spend bigger on the 9m - as you say it will be your go to kite 80% of the time in summer.

I ride a 12m (higher aspect so more like an 11m) 9m and 7m and there are not too many kiteable days I can't go out. Just wish i had more kiteable time!