I go back and forth, but right now I am leaning toward 'not planned.' Look at the state he left the house in---sheets pulled off the bed and thrown in the trash, bedding on the floor (which was likely the only thing out of place in this immaculate house), kids' beds not made, Wife's shoes and suitcase by the front door, purse and cell phone still in the house, didn't do anything to make it look like Wife ran off, her car is still in the garage with the car seats, didn't delete search history.
(The one suspicious thing to me is the deletion of his SM accounts a week before. (Not sure how we know that.))
We've suggested that he ran out of time because she got home late, but that's just a guess.
Alternative scenario: What if he was in such a panic that he loaded up the bodies in the truck and just headed out? (Ironically, the 40-mile route to the tank site is the exact same direction as if he was headed home to North Carolina.) For whatever reason, he stops at this particular Anadarko tank site. Maybe he was going to bury all three bodies, but he found out the ground was too hard, so he left SW in a shallow grave, panicked about what to do with the children, and saw the only hiding spot available right there---the oil tanks. He might have thought he would come back, fish them out of the oil and bury them or take them to a different location---just no rational thinking at all at this moment, only pure panic.
Do we know he actually went to work and didn't just call in and claim to be in the field? (I know---that's what AB says.) Maybe he goes by the office or does some work, then maybe he decides to head home to start cleaning up there, and before he can get back that meddling* NU shows up at the front door (which he can see from his Ring phone app).
And then he gets a call from NU, and I can imagine it going something like a certain scene from the very first Star Wars, when Han, Luke and Chewy shoot up a detention center trying to find the princess, the intercom goes off and Han says, "uh...everything's under control. Situation normal." "What happened?" "Uh...we had a slight weapons malfunction, but . . .uh. . . everything's perfectly fine now. We're fine....we're all fine here now, thank you. How are you?" (Doh!)
What do you think? It's a theory. About as good as any other theory at this point.
(* Scooby Doo reference.)