That looks like crabgrass from this distance. Weed-b-Gon does not work against grassy weeds. You would need a dedicated crabgrass killer.

My landscaper said it was Pennsylvania Smartweed and suggested I use Msm Turf to address it. I misread the mixing instructions and ended up using too much which destroyed the area I sprayed. Now the question is whether to address it in the fall by overseeding or wait until spring, any suggestions?

Absolutely address it in the fall. In fact, fall is about a week away for you, so take this time to plan your approach. Spring is a terrible time in most places of the country to seed grass.

I thought perhaps nothing would grow for a while in the area where I killed everything with msm turf.

Appears you have Annual Poa. Very aggressive and produces heavy thatch that crowds out good grass types. You can applie an pre-emergent to prevent the seed from the seed heads from germinating. Best I found was to De-thatch your lawn and use a crabgrass killer and pre-emergent to remove it. Next opportunity you have to re-seed, Fall or Spring (make sure you wait for the pre-emergent and crabgrass killer has dissipated ~ 3 to 6 months) then re-seed.