This app can generate your desired icon sizes, apply the proper scale and corner radius for macOS apps, toggle transparency, and even save as .appiconset with the included contents.json file.

The idea behind this application came about when I wanted to make the icon for Bubblemaker, because I found that creating an icon with the proper scaling and shadows was extremely difficult. Using SwiftUI to produce this enables me to get the result I am happy with every single time. It lines the shadows up properly, and generates all image sizes with the proper interal corner radius and scaling.