99% of crabgrass is annual, there is some perennial on the scene now. The issue with selective crabgrass control products is residual, which will prevent reseeding for 2-6 weeks depending on the product (Weed B Gone Max is 4 weeks until reseeding) and many of them are a lot more effective when used before the crabgrass reaches the 5-7 tiller (blade) stage. By now, it's fully grown and difficult to kill with selective products. You probably won't see anymore crabgrass germinating at this point, but those existing plants will continue to grow until the first hard frost. The issue with the existing plants is that they're thick enough to prevent good seed-soil contact so you'll either want to mechanically remove it (or scalp it) or kill it before seeding if it's thick- or widespread enough.
BTW Fusilade is effective on crabgrass but is generally used in ornamental applications. Tall fescue is about the only grass that will tolerate it; it will kill perennial rye, bluegrass and fine leaf fescue. It's not labeled for residential use either.
A non-selective (kills all weeds and grasses) weed killer, such as glyphosate (the active ingredient in Roundup, there are many generics out there now that it is off patent), allows you to reseed within 3-5 days. It is going to kill your desirable grass as well so you would want to minimize your overspray as much as possible. While relatively safe, compared with other pesticides, it is still a chemical application.
Tarping or laying black plastic is very effective, although completely non-selective and does take some more time. I do work with a number of organic lawn care guys who use them prior to fall renovations in heavily weeded lawns. Lay them over the area for 7-10 days, ideally when it's hot and sunny, and anything underneath will be killed. It can often kill off some of the weed and crabgrass seeds near the surface that have built up over the years, waiting for their chance to germinate, thus reducing their pressure next spring.
You've got plenty of time to reseed, all of September and most of October, so go with whichever method you're most comfortable with. Often you can seed through part of November if Mother Nature cooperates.
We can get into pre-emergent crabgrass controls or organic options if you wish.
But the key is to remember the thicker the turf, the less of a weed and crabgrass issue you'll have.