New Lawn advice & killing crabgrass

Wanted to get some advice on establishing a new lawn....
Bought home a couple years back in the Pacific NW, but didn't notice the problems I'd have with the back yard. There's a pretty hard clay under almost no topsoil. Last year, in an attempt to improve things, I dethatched a pretty thick layer of thatch, aerated, added some topsoil and overseeded. Unfortunately, I thought a crabgrass problem I had was small and could be overcome by the seeding and selective spraying. Didn't work. By Summer, the crabrass was pretty thick.
Decided to kill the lawn and start from scratch. My plan is to bring in some more topsoil and rototill into the clay, and then seed or sod in the Spring. Here's the problem.... the grass and crabgrass appears to be dead on the surface, but I dug a small trench just to get a better idea of the clay problem, and found the crabgrass roots thriving.
What's the suggestions for getting the soil ready for planting in the Spring? I could reapply Roundup on the crabgrass when I see it sprout, but I want to make sure it's gone. I was just going to till in the sod instead of getting a sod cutter since it appears the crabgrass goes pretty deep anyway, but should I rent the sod cutter anyway, just to eliminate as much of the problem as I can?
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