Yeah 16 minutes from Metro arrival to boarding is maybe a bit too close. You're relying on Metro to be very on time at that point. But it'd be interesting to see how long it actually takes you. My experience with the early TSA line at DCA is smooth even in the non-precheck line, and none of the gates are a long walk.

Edit: and especially since your plane isn't coming from somewhere else, there's no turnaround time for the plane to be cleaned and refueled. It's a high likelihood of leaving on time or even earlier.

I had a flight last Friday @ 7:30am. It was pretty crowded - long lines everywhere. I left early and took an Uber but there was an accident so it took 50 min to get there instead of 15. The line at curbside was long and slow - by the time it was my turn, they said I was 3 min past cutoff and wasn’t allowed to check in my luggage.

Take Uber. But check for traffic beforehand to avoid this mishap I experienced.

Do you live near any bus routes? It might take a little longer but generally the buses start earlier. My husband and I took the bus to Union Station for an early Amtrak once.