Editing the rounded-corner radius is a basic editing feature - and so necessary - that I’m amazed Miro hasn’t put it in place long before now. Without it, the rounded rectangles are very limited in their usefulness. There’s simply no default setting that will look good at all sizes, or match the tastes of the person creating the drawing.
Using Miro properly for the first time, I’ve realised why I’ve always thought miro boards look so unprofessional - it’s because of the radius on the rounded rectangles. It’s difficult to make anything look well designed.