Hi Mac!
Well...you'll probably get the best advice here on Thorn bikes, not Bokehs.
If what you are after is a lightweight and fast adventure bike that accepts wide 650B tires and does so with an aluminum or Ti frame, that is not going to be a Thorn.
I'm a little confused about your trip time constraints. If a week away is out of the question but 3-4 days is okay, that's asking a lot of your body to complete such rides in only two days while jet lagged. Hanoi to Dien Bien Phu by the most direct route is only 291km, but would typically require
at least a full day's air travel each way from Europe or North America (20 hours' flight time alone from Montreal, for example).
Or, is the problem one where the current bike is not comfortable for fully loaded week-long trips? Or not fast enough to complete the rides you'd like in the time allotted?
If you're looking to get into bikepacking with minimal gear and bike weight, perhaps something in the way of a lightweight Raven (Rohloff) or Sherpa (derailleur) build would do the job for you. They can be amazingly light if unemcumbered by racks and mudguards and equipped with optional forks and lightweight components. The default wheels would be 26in, of course.
At the other end there's the Audax, but it isn't really oriented toward this sort of all-road and off-road use. I suppose a Club Tour fitted with fairly wide tires might work to a degree, but it is more a general tourer in the traditional sense.
How is the present bike uncomfortable? Is it excessive weight of the bike as currently setup? Carrying too much? Perhaps peeling off racks and mudguards would help. Is it a matter of poor fit, or is it just not as fast as you'd prefer? Gearing or tires can account for much of that unless optimized for your needs. I recall you had a problem with uneven tire wear and mentioned it might be a Marathon Plus. If so, those are not very fast tires and a change might well make your present bike not only faster but more pleasant to ride. For comparison, my Nomad Mk2 weighs 20kg and uses Schwalbe Duremes and I find it good for riding unladen on 200m days on mixed surfaces with no problem. In comparison, when I tried Marathon Plus tires on a lightweight commuter bike, it felt like I was riding across mattresses...just a real slog.
650B wheels/tires are now an option for the Nomad, but it is a robust bicycle designed for carrying heavy loads (expedition work). It sounds like you are looking for a raid bicycle to do all-road riding with minimal loads as fast and light as possible?
Best,
Dan.