Schlumpf [its in the manual] wont tolerate fixie use..
KiSS method : , S3X, Dingle cog , bring along a wrench to loosen the axle bolt,
stop and change,
2 chainrings with tooth total F+R same , 2 more tooth on larger dingle cog,
2 less teeth on the chainring on the front. [F+R(a)] = [f+r(b)]

so chain length remains same , but drive range is wider than S3X on its own..

Perhaps a QR axle will grip enough to not need wrench in the pocket.

I have an AW3 and a schlumpf MD on my Brompton, its a wide ratio set

the equivalent to a 50t/20t crankset ..

What's so bad about freewheeling/coasting , its my favorite part of cycling.