You want to export files from DT and import them then into Notion. You also say that " that didn’t work" (regarding a MD file). I suggest that you create a simple MD file outside of DT somewhere in the file system (e.g. on the Desktop) and that you then try to import this into Notion.
I tried that for you: importing an MD file from iAWriter into Notion works. Exporting an MD file from DT to the desktop and importing it into Notion did not work. It imported 100% and then presented an error messages that was completely useless. The exact same file imports just fine into Marked 2 and Drafts (I didn’t bother to try any other apps, given that it is just a text file).
So you might want to take this up with the Notion people – they should know why they can import one file but not the other.
Without you explaining what you did, nobody can know if you’re trying “the right option” or not.