Braca's most flexible shaft comes in a 28 mm diameter. I have not tried it.
I know Lendal also carries reduced diameter shafts, but they don't have a true wing blade.
Nelo 550L, Streuer Fejna, Nelo Viper 55
Braca XI 705 EL blade, 17K shaft
Braca XI 675 marathon blade, 19K shaft
Braca IV 670 soft blade, 19K shaft