brompton 6-sp and the +12% 54t option was mentioned early on. you then get a top gear close to 110 with low friction. just one planetary gear to crank, and a 13 cog in the rear that is big enough not to cause noticeable drag or wear. good enough, and considerably better than draggy multigear hubs.

what to tweak then? well the rather high top gear is not backed up by gear ratios nearby. the jump between the highest gears is big, the top gear tends to live on its own.

you get a tighter setup if you instead buy a 3-sp brompton and make a 6-sp conversion; then the difference between gears is roughly 17 instead of 24 percent. to reach 100+ gearing you then also need a bigger chainring, preferably over 60t. up to 63t is possible without modification, if you wish to go further than that, the catch on the left side of the front wheel needs to be replaced by the s1e model, sits further up on the left fork. then the limit is around 72t.

or you can buy a 3sp kit for the brompton derailer. the ones on the market tend to favor range over thightness, i would go for a 13-15-17.

there are other ways, but these mentioned here are readily available, and easy to mount for a general public. i have myself a 2x5 13/14 cog sturmey-archer brommie from the 90s and a 2x10 dura-ace/schlumpf, both take technical skill and effort to materialize.

what you in my opinion should avoid are for one multigear hubs of models where planetary gears are stacked. some hubs like the sa5 use alternating planetary gears, that works better, only one is used at a time. in a hub gear, parts therein have to rotate really fast with a small wheel, and their already with big wheels troublesome friction gets augmented.

and also try to stay away from solutions with small cogs. already 11 is of little practical use, gives a noticeable drag and wears itself and the chain out in short time.

been there, done that.