@sarahwalsh You basically answered your question. You can also buy links and also aks other websites to link back to you. You would have to give something back of course in return but bottom line is you gain links from Medium, Reclaim Unlinked Mentions, Become a Source for Other Publishers, Get 'Best 10' List Mentions, Update Old Content, Reach Out to Journalists, Publish How To Guides, Offer Free Services or Tools.
These things can help you build your SEO.