Note added with a capture template, though I still need to manually assemble a bit.

(setq org-capture-templates
      '(("t" "Tumblelog" entry
         (file+olp+datetree bmw/tumble-log)
         "* %U %?\n:properties:\n:export_file_name: nnn.md\n:end:\n")
        ))

Less manual construction than a couple minutes ago!

I’ll have to learn some Elisp to make it work how it does in my head.