My 2 cents (about what its worth too)
I'm based out of RMN, so fairly familiar with the SFRA. I'd advise do the online training, carry the smart card, it can't hurt. I'd also advise not flying into the SFRA (I know, opinions are like..) but I say this because you don't really get a benefit for all the hassle. You are marginally closer to your ground destination, but will pay for that privilege. You won't "see" anything from the air because you will not only have to deal with the SFRA, but there is only a marginal chance you will get cleared into the Bravo (they even route IFR around it)...so you will be below that shelf and if you look, that's pretty low. The cool stuff to look at is all inside the FRZ and there's pretty much zero chance of getting in there (sure, background check, fingerprints..there's ways, but worth the time/money/hassle?)
Leesburg (JYO) has the cutout, it also has a couple of flight schools and the traffic in/out is heavy (and since you are going VFR, they will be out in force in the same weather you will be using). Parking is simple, FBO is nice, and you are close-ish to metro to get downtown. Tower "closes" at night here too (early, not midnight), that catches people sometimes.
Manassas (HEF) is inside the SFRA, but parking can get pricey (though is the spot for a hanger is you need one), close to VRE/Amtrak to get downtown, rental cars, etc. Towered and with several flight schools, also gets very busy. Its not uncommon to sit at the departure end for an extended time waiting for a hole in the traffic that tower will squeeze you into (roughly 2/10 times leaving there for me).
Warrenton (HWY) is just outside the SFRA, nice airport but not close to anything. You will need a rental car if you go here. No tower, but on Sundays the Flying Circus is going (grass strip just to the east) with some traffic (and they don't all have ADSB, but are brightly colored..nice show too).
Stafford (RMN), is just outside the SFRA, no tower, flight school has about 3 planes, so they are there, but not really clobbering the pattern, rental cars available on the field or short Uber/Lift to the train station. (I used to commute to DC, its about $12 from Fredericksburg to DC, easy switch to metro to get you everywhere you want to go). Parking is dead simple and the airport is pretty friendly.
For all the spots where you are thinking about train, VRE runs into the city in the morning and out of the city in the afternoon/evening..Amtrak does its own thing so YMMV.
Driving from RMN to DC is about 45 min unless someone goes sidewise on I-95 (happens). HEF to DC is about the same on I-66. JYO is further out than HEF. Parking in DC is challenging and watch out for 'right on red'...
If you haven't already found them
VRE https://www.vre.org/
Metro https://www.wmata.com/service/rail/