A wider tire at lower pressure could help.
I ride a Tikit and I believe the Greenspeed Scorcher tires, which are 40x349, help a lot in smoothing the ride while still being one of the quickest tires available in the size.

As discussed in this thread, some have had a good experience using the Scorchers on a Brompton, and some saw fiddling with it not fully rewarding.

Good luck,
Roger