There is no such thing as “fast easy evac” onto the GW Parkway after the fireworks. Seriously, if you drive and park along the parkway (legal or not), you’re in for a long sit in your car. Throw your bikes in the car, go park in Crystal City, and then roll up the trail to whatever spot you like. As noted above, you’ll be picking your way through crowds on the way out, but you’ll be in a position to just jump on 395/110/whatever isn’t GW Parkway after.
(If you’re headed there early enough, the parking lot on the Pentagon side of the footbridge to LBJ park may be a good idea. But it’s probably one shared by many other Washington area fireworks fans. It’s a good staging point if you have picnic stuff you want to keep in the car, and it won’t force you onto GW Parkway later.)