Have you considered going with a multi speed rear hub similar to Sturmey Archer that uses both the internal 3-speed hub and a 9 speed rear derailleur. I have one trike with the no longer available SRAM dual-drive that expands the gear range on both ends. It's the least expensive way compared to multi-speed hubs like the expensive Rholoff speed hub and you can use a standard cassette to get the gear range you wish.