A.
If you haven't already, check out the Workshop Rotation Model at www.rotation.org
I have been in a church that used this (1600 members - large but not as large as yours, obviously) and I found that it worked really well. It was easier to recruit teachers because they only needed to commit to one rotation period at a time - 5 weeks in our case - an learn one basic lesson with some modifications for age groups. We had fun decorating the classrooms, too, and the kids seemed to really enjoy it. Also, by being exposed to the same subject for several weeks in a row the kids really got to understand it in different ways, and the kids who didn't come to church every Sunday didn't miss entire concepts while they were gone.
I'm in a much smaller congregation now where this doesn't really work for us, and I miss it!
Good luck with your task - it's a big one!