Write the posts on a self-hosted site, then syndicate them to Medium and share those links on social media. Another option is to use a custom domain with Medium[0]. That way, if Medium goes out of business, you could just set up a self-hosted replacement blog with redirects for the URLs for the posts you made on Medium. 0: https://help.medium.com/hc/en-us/articles/213474588
You can also buy managed Ghost, WordPress, etc for not that much. I think both pro accounts allow custom domains and exporting.
If you're a developer, or comfortable with code, static web site generation (Hugo, Jekyll, middleman, etc) might be an option too.