Do they have on-site parking? If not, where's the best place to park for cheap / free? And what's the traffic generally like in the area? I find city D.C. traffic to generally be a nightmare, but I'm from a smaller town, so...

There's a parking lot owned by the venue that you pay for, and there's street parking in the area but there's no promise there'll be a spot, especially on show nights

9:30 clubs parking lot typically requires that you purchase it online in advance. It's $20 and worth it. Just go to the website and find the show you're attending. You'll see a spot that says "parking" which will allow you to buy a spot. The lot is secure and right next door. I drive in from West Virginia and this is what I do every time.

Public transportation in DC is dang good. Metro trains and busses are MUCH better than they were just 3 years ago, and the electric scooters pick up where Uber or Lyft leave off. Taxis are atrociously expensive, relative to the other options, and are usually sheitholes of safety.

Yeah its best to take the train parking is limited

Best to park near a metro and take it to the venue