any year after 3rd, 4th, or 5th. moving after 5th should be okay actually, prior friends are also going to new different school, adjust to being brand new during 6th grade at smaller school, then some friends and acquaintances from 6th will presumably go to the same middle school for 7th (plus more kids to befriend if last year was difficult for some reason).

The sooner the better. Don't wait until 7th grade. Middle schools in Farifax are HUGE like over 1000 kids for just two grades. Even students who have friends and relationships from elementary school get lost in the shuffle. That seems like the worst idea. If you find a home and can afford to make the move, do it next summer. Much easier to make friends in elementary school than middle.

Now. I would move now. We moved from Alexandria to fcps in 2nd and 4th grade. Kids were young enough to be kids and before adolescence. Ds started middle school and had several friends already.

You don't want her joining a cohort of kids who have been together for 5-6 years just for 5th or 6th grade. That's a hard thing socially. Nor do you want her to go to three schools in three years. I'd move in 3rd or 4th or plan to stick it out until high school.