Thanks for sharing the idea and implementation!
I have played around with the same structure some more and could achieve rendering of "checkboxes" when using this alongside the option hide Markdown: More in the Extra Markdown Plugin:
I would like to implement proper ``it! support in Joplin.
I have created an issue for that here if you want to join the discussion or for anyone to have a look at the current implementation: [Feature] Implement optional [x]it! support · Issue #44 · personalizedrefrigerator/joplin-plugin-extra-editor-settings · GitHub