Here, my sweater had slits and I only tucked in the front. By doing this, it ended up being tucked in beyond the slits, which helped the back part blouse under on its own.
I kept the sweater completely smooth in front of the skirt, and let the gathering happen at the small of the back. I bloused it out a bit to make space for the gathering.
I only tucked in a couple of inches of this thick sweater. Any more, and the drape would be lost and with it my shape and my comfort.
This sweater was fully tucked but because it's lightweight it didn't add bulk. By gathering the bulk at the small of the back, it still achieved a high-low effect.