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.