WP site:
Change it from showing the excerpt to show full content on first page. If you want to show just a part of the article (as you should do), use the "read more" tag (or block) as a delimiter. Write some posts on your expired domain, related to the theme of the domain or just take some old content from the Wayback machine.
Place your link in the first part of your post, so you can add the "read more" tag after it. This way, the post will be published to the first page and it will also include your contextual backlink. The link will also be included in the post, of course.