Sorry, I don't understand

I need post.content as a raw markdown instead of HTML DOM

eventually something to HTML -> text

I guess you are on your own then. Look at the code, it is quite straightforward.

You can see it is the markup plugin that actually reads the file and at once turns it into HTML https://github.com/cryogen-project/cryogen-flexmark/blob/master/src/cryogen_flexmark/core.clj So you could e.g. make your own impl. of the Markup protocol that copies what cryogen-flexmark does but also adds the changes you need.