OK, after reviewing the forums topic Indentation issue with BodyMacro expansion I got the idea...
let newBody = CodeIndentingRewriter(style: .unindentSpaces(4)).rewrite(body).as(CodeBlockSyntax.self)
and then using newBody ?? body in what follows.
See the implementation in the PipelineStepMacro (to be moved into the Pipeline package if proven in practice).
Thanks again!