one suggestion.. consider buying another crankset, & BB.. 110BCD with a double chainring ,
perhaps a 36/44t chainring combination.

then you have 2 ranges of 6.. , with no FD, the range change is manual .
keep a shop rag handy.
[Todd, owner of Portland dealer for Brommys, used that setup on a Ore, Nor Cal coast tour]

considerably lower. the Swiss mountain drive crank by Schlumpf..
A second, planetary instead of the 2nd chainring, turns a 50t into a 20t, in low range
but chain stays on the 1 chainring.
There is a model made for Brompton.
IG torque transfer is tidy, a knob sits on top of the BB just ahead of the rear pivot.

set you back in the wallet, though..