I have really narrow heels. My foot is more medium volume on the whole, though (higher arches and regular width forefoot) but I have indeed experienced that slipping forward problem in sandals in the past, so I know whereof you speak. Still, what works for me might not be good enough for you, but might be worth a lookout just in case.
I got these Fitflops on the advice of a friend who said they were helpful for plantar fasciitis (which I have and which was acting up.) Most Fitflops do not work for me as they are too wide, but these have adjustable buckles. I took my smaller size (the larger size was too floppy!). I've been wearing them for a month and love them. Wear them all day, about 10,000 steps most days. Obviously, they're a super casual shoe only.
The Sofft (which comes in multiple colours) might be too wide, but it just might work. It's a super comfortable sandal and a little dressier than the Fitflops.
The Eccos might also be too loose on top; they're verging on that on my foot but not in humid weather when my feet swell a bit. I find them not quite supportive enough in the arch, but they're nice and cushy and very lightweight.