Hah! That looks like my friend's driveway. I used to do a hill much worse than that almost daily for 7yrs. Don't have the geo data to pull up(shame I'm not still assigned to NGA),but from the pics DC is either equal or worse. Note,I also live at the second highest point in DC.

15-17" is really super low. Like,you'd have trouble going fast enough to maintain your balance. Lowest gear on a standard MTB setup(22/34) is 16.8";my '05 BBU has those gears,and I could never imagine using it unless I was pulling a 100lbs trailer.

Schlumpfs are crazy expensive,like $600+. So if you're going to go that route,it might be better to spend the money on another folder with a derailleur drivetrain you can easily modify(cassettes and rings are comparatively cheap).

Have a MTB,or know anyone with one you can borrow? Ride one of those up the hills,take note of the gearing,and plug the numbers into a calculator to see what you really need. Then check into your options. Does your Brompton have the new standard 130BCD ring crank? If so you can go alot smaller on your front ring for cheap. And if not,swapping cranks is still going to be cheaper than either a Schlumpf or 6spd conversion.