I you have a real thatch problem, which isn't seen that often, I'd
de-thatch first. Having a layer of some dead, brown decaying material
is normal and not a thatch problem. Then I'd aerate and use a slice
seeder at the same time. One problem you will have is if you have 80%
crabgrass coverage, there is still going to be a lot of crabgrass there
trying to compete with the new grass. So, instead of seeding in early
sept, I'd probably wait till last week in sept. With falling temps,
the crabgrass will be on the decline by then.

The other choice is you could kill the crabgrass now with Aclaim. The
other question is, with that much crabgrass, what's the rest of the
grass like? If the lawn is a complete mess, with lots of weeds and
undesirable grass, the best solution may be to kill the whole thing
with Roundup and reseed in early sept. You can reseed a week after
roundup, but I'd give it more like two weeks so all the existing stuff
is dead.

Just out of curiosity, what did Lawn Doctor do? Did you get them too
late? Only one application of pre-emergent done at the right time,
which for your area would be late april, when forsythias bloom, should
keep this kind of thing from happening. You might have a little bit,
but shouldn't have an 80% crabgrass problem.