Got a Mountain Drive for my Brompton, reduction gear of 150%
so the 54 acts like a 21.6t in the reduction gear .

Given the 15/54, is close to the 13/47, I'm thinking of double 'chainringing' the Mountain drive.

then half stepping the AW3/BSR , from the front, with a 13/47, 13/54,
rather than the BWR 13,15 6 speed in the rear ..
top 6 it will matter , in narrowing the ratio in between space. bottom 6 less so.