Check out hotel restaurants first. They are all open and some might have buffets. I love seeing a nice buffet at Thanksgiving, with someone carving a turkey and a prime rib, and then all the fixings! Enjoy, OP!
I'm almost embarrassed to admit that I have done this once with my in-laws on a Thanksgiving trip. And you know what? It was fine, and we all ate a bunch and survived!
Mount Vernon Inn Restaurant is doing a buffet. The Fairmont Hotel in Georgetown did a buffet last year, I think. It seems like a few other places that have done buffets in the past are just doing family meals to go now. Maybe buffets are a bit too germy? Not sure you want to travel any further outside of DC than Mt Vernon if you are just visiting, but I am pretty sure the Salamander and Lansdowne resorts do buffets.