You're a brave man for even going to Thailand at that time of year - a hell of a lot of expats head out!!
I've never driven in that area but I'd take a guess you'll be OK. Yes, I would expect it to be busy but nothing like the arterial routes from the south. I have though, made the journey from Bangkok to Isaan at both Songkran and New Year - absolutely horrendous and something I'll never do again. Suvarnabhumi to Khao Yai - 9.5 hours against the usual 2 - 2.5. I think you'll miss the worst problems by flying to Udon.
The busiest routes will be those from the capital up into Isaan on the evening work finishes and the evening before the return to work (your Mrs will be able to find those out) - i.e. Highways 1 & 2 and highway 24 from Korat onwards. Obviously the traffic will diminish as people reach their destinations. The traffic, although hideous, isn't really as heavy as it seems - the delays caused by literally hundreds of accidents cause a lot of it.
As I say, I will never travel from Bangkok to anywhere on a major national holiday again.