Sturmey Archer AW3.. Brompton has used them from the beginning , and currently..

28 hole hubshell , narrow axle options .. in spare parts, SJS, UK, has good stock ..

the steel trigger shifter is rugged and reliable. brompton placed it on top of the M bar ..

straight bar it can go underneath, to use index finger, up and down shift..

IDK what complexity you speak of .. I just read the service manuals ..