You can get handlebar raisers, but at least with a Brompton, fitting non standard parts can cause problems with the fold, and very likely this applies to other folders too. Finding somewhere that has a few to try is the ideal way to go. Dahons and Terns started from the same roots but I imagine they aren't very similar now. Some random no -name folder might fit your needs out of the blue, but it's hard to know without trying.
As an example, the Dahon Vitesse HG7 which I had felt cramped and had a hard ride and perhaps due to the telescopic stem managed to feel flexy too. Yet the Viking (which was probably some badge engineered product of an unknown far eastern factory) Safari had a one piece, very long stem and the reach was a little further than the Dahon. It had some flex but despite the wheelbase being only an inch or so longer, gave a more comfortable ride. There were no names on the rims and hubs but the transmission was Shimano. It seemed well made, and nothing broke or came loose while I had it. I regarded it as a small wheeled bike that could fold if it needed to. It wasn't particularly fast, but went well enough. I did enjoy riding it.The Brompton in contrast is all about the fold, and yet manages to go very well for a bike with 16" wheels. Perversely, I quite enjoy that, too.
If what's needed is a bike that fits first of all, then can keep up with with all the big wheeled bikes, you will probably have to accept that you'll need to kiss a lot of frogs before you find the handsome prince among folders that does what you want.
. It would need a slightly offset hinge to move the rear wheel to one side so the front wheel could telescope to the position beside (or a main tube which isn't central, or an offset wheeltrack). Hmmm this could require several prototypes.
