My Mountain drive crank gets me an 17" low gear with a 54 /15 BSR .. the reduction gear is as if the crank has a 21.6t granny gear
when kicked into low range .. Yea it comes in at about $600.

3rd to 4th double shift is Quick since both gears are internal , at any speed even stopped
So 4th to 3rd bogged down on a hill is an easy shift..





There is also the option of fitting a double crankset of conventional type, and use the heel of your shoe to dislodge the chain from it's Big Ring .

that could be more affordable.. You already have the chain tensioner