Even though we are much closer than you, we still haven't hit upon the perfect plan of when to leave home or arrive at the Fort. When we stayed at the All-stars, we would leave after work and drive straight through. When we camped, we tried leaving work a little early so it wouldn't be too late pulling in but that didn't seem to happen and it was always midnight or later.
We then switched to leaving home on Saturday morning but it seems we can't resist those stops that will "only take a minute" and before you know it our lunchtime arrival ends up being right around dark.
For our October trip, our plan is to leave after work but stop no more than 2 hours from WDW at somewhere we can just pull through and plug up the electrical. I would like to arrive at the Fort no later than 8 or 9 in the morning.
Tell your DH to drive safely as I know he must be getting tired. It sounds like you have a wonderful family!