I have 2015 Norco Bigfoot fat bike which has a Shimano HG-20 11-34T 9 sp cassette a with a Shimano Alivio RDM 4000 SGS 9 sp Rear Derailleur and a Shimano EZ-Fire EF 51 9 sp Rear Shifter. As I live in a particularly hilly area and intent to most ride this bike off road along animal pads in the bush I would like to reduce the gearing by having a 42t cog on the cassette. From reading I understand this can be done by - removing a small cog and installing an 42T extender cog in the cassette, - by replacing the current cassette with a wide range 9 sp cassette (hard to find),or by - changing to a wide range 10 speed cassette with a change of derailleur, chain and shifter.
Please provide advise as to the best, most economical option. Could I install a 10 speed cassette and just use 9 cogs?