I would probably use a message field because your goal is for copying and pasting.
The easiest way to do this is to create an acf/prepeare_field filter for the message field.
The message field does not need to have any value to begin with. In you filter you would get the values of the other fields and then and the values format $field[‘message’] as you want.
This will only work after a post is saved. I is possible to do this as values are added or selected, but I can’t give you any examples of doing this. You’d need to add custom JavaScript and do quite a bit of JS coding to get the values of the fields as they are updated and then update the message field content.
