Brompton took an alternate approach in their rear 'derailleur',
since the change is just A/B, rather than over complicate a simple task, .. k.i.s.s.
they engineered the pulley to incorporate guide flanges, to keep the chain on the pulley.
then a simple fork like lever pushes that pulley, itself , directly sideways.

I use the Sachs 15 t, it fit my AW3.. 3 teeth are shorter for a shift gate.

Some custom builders, like Steve Parry in the UK, have made a bracket to put a FD on
of a braze-on type, and then an additional lever is added to the handlebars.

that is more practical as a range shift, rather than a half step,
the BWR hub is wider ratio than the AW3/BSR hub, ant the 13/15t cog set half steps that.

Older Mk3 bikes got Sachs to make the 3 by 2 hub, its a narrower ratio spaced hub,
its 'cassette' uses the AW type cog ,
spares of the thinner cog , for 3/32 chain, can be useful. Vs 1/8" , thicker cog..

I sense their use of plastics, chosen, mold design for the materials,
are well designed, and robust .

the chain-tension arm is a pretty solid piece..

I have a M3L, I use a 2 speed Schlumpf Mountain drive crank
instead of a front derailleur,
To increase my climbing capabilities, 6 speeds in a widely spread pattern
the 3rd to 4th is both a hub gear and a crank range double shift,

but being both planetaria, that shift can happen at any speed,
so the bogged down in low while in high range, is no problem,
to shift to .. high in hub, low range in the crank, the next lower ratio.