Hi and welcome to devnet,

If you want a full static application then you have to also build all dependencies as static and since you're on Windows, you also have to take into account the Visual Studio Runtime (since you are using a Visual Studio Qt build). Note that this is nothing Qt specific.

No, you don't have to delete anything, you can use several builds of Qt in parallel without any problem. How fast your build time will be depends on the machine you have.

Developing applications and the type of builds are two completely unrelated topics.

Note that using a static build of Qt has licensing implications.