How to change enum to struct using FixIt

I'm delving into the macros topic to enhance the developer experience with my macros.
I envision a future where the Xcode Fix It button empowers developers to make source code changes effortlessly. For instance, it could intelligently switch an enum to a struct if my attached macro is designed exclusively for structs.
Now, my challenge lies in navigating the syntax tree to identify the appropriate struct declaration syntax for the FixIt.change property:

fixIt: FixIt(
    message: CustomError.invalidDeclarationType,
    changes: [
        .replace(
            oldNode: Syntax(declaration._syntaxNode),
            newNode: Syntax(???)
        )
    ]
)

How can I locate this syntax?

Additionally, I attempted to debug the macro using tests to explore the tree and find what I'm looking for, but encountered an issue — my tests failed without triggering any breakpoints:

let testMacros: [String: Macro.Type] = [
    "CustomMacro": CustomMacro.self
]
final class StructTests: XCTestCase {
    func testEnumToStruct() {
        assertMacroExpansion(
            """
            @CustomInit
            enum Caller {
                
            }
            """,
            expandedSource:
            """
            @CustomInit
            struct Caller {
                
            }
            """,
            macros: testMacros
        )
    }
}

What could be causing this discrepancy, and how can I resolve it?