Hi Neighbor! I'm another DC-based shopper checking in.
I've also been at this for about 6 years, but only part-time. I average 8 assignments per month, mostly here in DC, but I do venture beyond the District boundaries to pick up assignments elsewhere the metro area or beyond while traveling.
I am signed up with 35 different companies, but 75% of my mystery shopping work is for just 6 of them. There are lots of opportunities here, covering a wide variety of geography, client type, and shop profitability.
Welcome to the forum. I probably don't need to tell someone from around here this, but don't let the occasional bit of snark scare you off. Most people here are super helpful, including (especially?) those who like to give one another a good-natured hard time. There is a LOT of great information to be learned here, from seasoned professionals and relative newbies alike.