Northern Virgina, you have plenty of time.
We are in SE PA and I will aerate and seed for many more weeks with success. I run two crews aerating and seeding every year.
My suggestion, and I do several of these a year. You sprayed Roundup once already. Wait 7 to 10 days and spray again to get anything you may have missed or is not completely dead.
Assuming what is dead will not be that tall I never bother with mowing it down or dethatching, sure you can go ahead and do this but the extra work is not needed. Give it a day or two then aerate the living hell out of it. Seriously, take all the labor you saved by not dethatching and mowing and put it into aeration. When this is finished aerate again. Chew it up thoroughly to create a good seed bed.
Apply the tall fescue at 10 pounds per 1000 square feet. Unless you have grading issues you most likely do not need to add topsoil. However, if you do have a good source for organic matter, a light topdressing will do wonders for improving the soil.
10 days after seeding apply starter fertilizer. Apply a quick release fertilizer again around early November and you will be good to go.
At some point you should get the soil tested to determine your pH. I wouldn't just assume you need lime and guess. the test results will tell you the correct amount needed.
Hope this helps, Ive done it this way for years with great success.