Some folks may not know this but Logseq has a built-in whiteboard feature too that’s also FOSS. I use it all the time to mind-map new blogposts and newsletters.
In Logseq the starting page is always the journal page for the day. This allows you to build up content without worrying about where it should go. Once you have something you feel you can run with, then you can move it to its own page.
EDIT: more features enabled by Logseq’s block-based (bullets) architecture over on Mastodon.