Originally Posted by
Antifriction
Optimum depends on budget (# of speeds) & terrain. For life on the cheap in hilly Vancouver, I've given my 7-speed Dahon an 11-34t cassette which pushes the jumps to get 309% range, & a 42t chainring. 25-76" range gets me up almost all hills at the price of coasting on long descents. If you can afford Microshift Advent X (budget 10-speed, 11-48t, 436% range), you can build an all-world 20" bike: 48t chainring, 20-87". For less than 10-speed, and given that you're an online business, I'd suggest making chainring size an option - maybe cassette as well. Folks on the prairies may be best served by a corncob.
Maximising the range is one thing but not the only thing to look at. One of the big thing to consider is progression. for example, like you I run a 11-34T on 20" wheel but on a 44 rather than 42 and it not far from yours however, mine is 9spd making the gear spread closer than yours and that is important in therm of comfort. When I run the helios on 7spd 11-32, I found the changes horrible as the steps between gears were too big.
As my folding gravel was a budget built, I purposely set the gear spread to match my big gravel and start from roughly the easier compromising the harder gears however, the gear progress is the same on all my bike so no nasty gear change/jump when going up hill of on the flat... down hill does not matter.
Note that on a budget, the cheaper is previous year road racing groupset. TT racers like to have the latest piece of kit and sell their old one on ebay etc. these kits on a folding 20" bike tend to give commuting bike ranges (27.5/29" wheel -> 20").
2nd hand shimano 105 on a folding bike is affordable
Note : 10-speed, 11-48t will not fit well; yes the cassette fit but it requires a long cage derailleur which will drag on the floor with a 20". I test fitted a acera long cage (with 11-36 on my daugther 20" frog52 (seeable on GCNTech Show @ 17.34)) and it was 12mm from the floor (midd tyre level). I tried a new 2020 Sora medium cage, it was a little better, ~50mm clearance... so I went for a new 2020 short cage Sora (it is longer than 2015 sora short cage) , and I got the right clearance with a 11-34T cassette