gear range

Hi, I am trying to get a custom frame built for a Bafang M510 or M560. I'm designing this bike for many purposes, including remote touring, mountain biking, and commuting. My goal is to have as wide a range of gearing as reasonably possible. The bike should be comfortable to ride with no motor...

what is the greatest range you can find on a 6 speed cassete....sorry freewheel hub cassete like the shimanos mega range etc..... am i limited to just 14 to 34t....where and how can i get a bigger gear spread....thank you.