A 20" Dahon with racks, eg Mariner (Al frame) or Speed D8 (steel frame) sounds like a reasonable compromise and not too expensive an experiment if it doesn't work out. Bickerton (I think Al frame) fills a similar niche in the UK, although only the brand name is original. I recommend getting one that has the luggage bracket welded to the frame in front of the head tube as that increases your luggage options a lot.

A Brompton is better for multimodal transport as its fold is much smaller but may be a bit compromised on the tracks that your job likely takes you down.

I have taken my old Dahon Speed down some pretty rough terrain on touring tyres. Not too fast though obviously.