Presently in the shapes you can only pick a rectangle and a rounded rectangle. The corners of the rounded rectangle distorts at anything beyond a 1:1 ratio, so there’s no true rounded rectangle available.
However, on things like a button we have the ability to change the corner rounding value. So I propose you take the rectangle and add the ability to round the corners, then replace the current “rounded rectangle” with that same shape, but already defaulted to a noticeable corner rounding, which could then be further adjusted to whatever the user prefers.
This would make the shapes consistent with your other objects and give some nice options for UI design work.