At 6.5 , a booster is a completely appropriate choice of restraint. The Apex 65 just got some pretty good reviews when used as a booster too - just double check and make sure YOUR child fits well in it in YOUR vehicle. Ideally, you want the lap belt across her hips, NOT stomach.....if the belt is riding above her hipbones, the booster is NOT appropriately placing the belt.
If you desperately want to harness longer, there ARE seats on the market that have higher harness slots. The nautilus, frontier, regent, safeguard, signo, como seats are all going to give you another .5-2.5 inches of harnessing...now since I see you have other kids to potentially hand that set down to, getting another harnessed seat could even make financial sense in the long run for your family....but again, at 6.5, a booster is a completely acceptable (and there are some who even argue BETTER!) choice.
but yes...i know how you feel....