I don't really see how other bikes can be convenient when taking in and out of shops. I have a 20" and 16" Dahon and in some ways they roll better than my Brompton (with rack), however the rolling is a sort of balancing act that requires attention and the folded size is cumbersome, it's not a happy situation in shops and buildings generally. The Brompton is really too heavy but I have the option to use it as a cart and wheel into shops or to fold completely and carry it, and put the luggage over my shoulder (I added a shoulder strap to the folding basket). So for me I figure bikes other than the Brompton will require locking outside some shops which means the additional weight of a 1.6Kg lock, the combined weight being heavier than the Brompton.

One option would be to be to use an S-RF5 (W) hub on the Brompton combined with the derailleur for a 10 speed setup, and lose the Schlumpf. The hub is only a little heavier than the 3 speed. If that gearing works for you you could upgrade the rear triangle to ti and the overall weight of the bike would be reasonable-ish. I've said it many times elsewhere that the notion of offsetting the weight of high tensile steel with titanium is ridiculous, Brompton are resting smugly on their laurels and are yet to bring their product into this century, but the utility of the fold is unique.

I'm not sure about gearing options on a Mezzo, they're light and compact so might fit the bill?