It sucks that you can’t do this basic thing and change the corner radius of a rectangle as it gets bigger and uglier. Workaround I’m using is to create the shapes /corners you want in Lucidchart, Microsoft Visio or Draw.io (the one I used) and export them as .vsdx files. You can import these using the import button at the top of the shapes panel (click All Shapes) in Miro and then copy/paste them into your project. You can still change the styles, colours and line thickness etc and scale them without pixellation. The initial bit is a bit clunky but if you create a bunch of shapes you might want in one go in the 3rd party software and different corner radiuses etc then at least you can have a ready made library to draw on when you need it. Someone better than me could create this library and share it here as a board or example project perhaps…
Considerations: Changing the direction of the shape in Draw.io for example might have a negative effect on connecting points and orientation of text. I don’t have Visio but it might be a better option as Draw wasn’t that intuitive?
Corners will stretch if resized along only one axis in this method so different shapes and sizes with different corner radiuses that are close to what you need would be the best as
Someone else posted a video where you can make simulate the effect of corner options within Miro itself but it only works for solid shapes and not outlined ones as it’s effectively made by grouping shapes together so you would see the overlaps.