FWIW, we parked on Sunday at 28 K St. SE (between South Capital and Half) in an underground garage that we booked via Spot Hero. The reservation was from 12:05 to 5:05 for what was originally supposed to be a 1:30 PM game. The garage is a bit difficult to get to as it's down an alleyway off of K street that isn't well marked, but the SH app had a pretty good picture of the entrance and noted that it was down an alley. The cost was $25 which was about the lowest we found in the area.
The game ended, IIRC, at around 6:30 PM. By then the SH app said our reservation had expired, and the app also said that the garage closed at 6 PM, so I was pretty nervous walking back to my car. When we arrived at the entrance to the building on the corner of L and Half Street, there was a security guard catching people heading toward the alley and directing them into the lobby of the building to get to the elevators. Once we got to our car and exited, there was an attendant at the exit copying down SH numbers to allow us to exit (they don't have a QR code scanner).
All in all it was a bit stressful, but worked out well.
Previous to that, we parked at 100M (corner of M and 1st) for our last game. It was also $25. For that game, the 1st St entrance that SH directed us to use was blocked due to construction, so we had to loop around the stadium and use an alternate entrance on M. When we got back to our car (Tuesday night game), the garage was closed, but fortunately the exit door opened for a car that was leaving as we arrived, so we walked in and headed for our car. The attendant seemed exasperated that people were coming into the garage to retrieve their cars (IIRC, it was around 11 PM), I guess he wanted to go home.
When we exited we had to give him our Spot Hero reservation number. He then opened the door for us to leave. And as he opened it, a flood of people walked in who had been outside trying to figure out how to get back to their cars. It was a mess.
So...moral of the story, SH works but with some challenges. It might be worth the extra few dollars to park in a Nats-operated lot.