As for design process itself, there is a simple fix for rounded corners. You need to convert revertible rounding effects into an outline, enter the shape editing mode and manually move curve handles closer to each other.
Optical Effects in User Interfaces (for True Nerds)
Slava Shestopalov
57K136

There’s also a Sketch plugin called Egmont that does creates superellipses/“squircles” automatically: https://github.com/interfacemarket/Egmont-plugin

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