That’s not a bad idea. Thought of something else that I thought of too – Pandoc is a document converter (Windows/macOS/Linux) that can be used to take the html files generated and convert them to markdown.
What works well too is to open the HTML file from the “Outputs” directory and copy the HTML from the beginning of the post content to the end and past it in online converter “ConvertSimple” and then past that in Publii’s markdown editor.
Guess the only caveat then is to create a new post with the above methods in a markdown editor and copy all the metadata from the original WYSIWYG post, especially the slug/URL etc and then once done trash the WYSIWYG post.