I think the kind of V that flatters depends on a bunch of body type variables, including length of neck, length of hair, width of shoulders, size of bust, and general frame size.
I only have one V neck cardigan (just purchased) and the V falls just below the sternum. I'll never wear it without something underneath, obviously. What goes under will likely be crew necks, button ups, and the occasional turtle neck. Like you, I may also use a scarf to fill in the space.
I also have one low V pullover. I find it tricky. I usually wear a crew neck shell underneath it. I would like to wear a tank and sometimes do so, but I'm not crazy about the proportions on me.
I think when you have a wider shoulder line plus a smaller bust -- and especially if you also have a long-ish neck (as I do) and short hair (as I do and you used to) there's a tendency for that chest area to look sort of ...I don't know....like a broad plain? That may be why we tend to reach for a scarf to fill it in a bit.
Heck, I just added a scarf today to a slightly looser CREW neck!! It was partly because it was #($*%* cold here. But it was also because it just looked like a weird open space there somehow with just the sweater. (Scratches head.)
Angie only wears Vs that cover the neck-shoulder point -- in other words, she is looking for a very NARROW V (the depth is usually correspondingly shorter with these). That's for a stand alone top, though, not a cardigan. I don't know if they make cardigans with a stance that high.
I think a small, narrow V looks great on me, too, and I really enjoy wearing it. But they're not easy to find.