The biggest problem I have with dynamic is the lack of elasticity in the algorithm.

Like chromestar said, practice :)

One way I do morphs like that is to use the "sag" morph brush. and make the "gravity" direction the direction you need.

You will also use flatten and smooth. A lot of the work will be in making up for the lack of brush shape control. It's round. That's it.

It would be nice to be able to use an ellipse for "between the breasts".

Sometimes, I use a magnet then tweak that result with the morph brush. You can also go dynamic, then morph on top of it. But the lack of elasticity in the cloth sim means you're never going to get what you want without hand morphing

This took just a couple of minutes, I started with sag, made it +x direction so I can change camera and it still pulls "front". Then I used flatten, smooth, and tighten, usually with small brushes and small values, (but cumulative). All at subd 0. Check in the subd you're going to render, though. I turned the transparency up so you can see the body. I made no effort to fit beyond the area I was morphing. Is this the effect you're looking for?

You could create a magnet set to do that. Or a donor morph. Certainly, to get you started. There's no built-in magic, though :)

Dynamics with elasticity would probably do it. I even tried to simulate it by setting up a dynamic sim, then having the breasts grow during the sim hoping it would stretch the cloth across the valley. Didn't work. Still think it should. Imma try again :D