If I make another 406 folder to have lower gearing from OEM like my 2X, I'd probably go 1X; It's hard to find that older style FD adapter, newer ones are more fragile, needing to grind the FD inboard cage to clear, MS FD having ridiculously stiff spring (Shimano won't fit adapter, more robust linkage interferes), etc, going 1X will be simpler.

Now with a 349 Brompton or the plethora of clones, I might need an IGH to get the high I need, and 400% gear range (same range as you noted, that's very close to what I run right now). With a 10T high cog and good sized chainring, I think possible, but a really big 1X cassette, doubtful, at least on standard 'Brom tires. The big fat tires you run on 349 and 305, perhaps yeah. I don't expect to build up a clone, I only think I would buy one if I visited Asia and bought one to credit card tour and bring back home. I'm retired so not a commuter, I just don't need a "frequent folder" here. Occasional folder, yes. On Amtrak, invaluable. Never tried yet, but I think I can bring my bifold on the light rail here without folding (a pain with panniers, which I need for shopping), but I haven't had need yet to ride the light rail yet, though it's expanding. Someday perhaps. Right now I need the exercise.

My bifold is still quite the conversation piece, even in a big bike town like Seattle. I'll see some Bromptons, one early BF All-Packa, but most folks have an electric or rent them. Only when riding the bike routes in late afternoon commuting hours, do I see folks on serious manual bikes, durable setups, all 700c but good gearing, rack, waterproof pannier, fenders, etc, UTILITY. I really enjoy seeing those riders. But I have yet to meet one that knows of Bike Forums.

I'm very lucky, all that I need, food shopping, doctors and dentist, goodwill store, everything is within biking distance. My room is small but the location is good. I just wish there was an elevator.