You can (sort of) create draft posts on Mastodon by writing a post, then set the visibility to "Direct" to send it to yourself as a DM and click "Toot!". It will be stored in your Direct Messages folder but will not be visible to anyone.
When you're ready to post, just go to the DM, select "delete & re-draft", edit it until you're happy with the contents, then set the visibility to public and click "Toot!".