Again, if you’re worrying solely about SEO, then you’re looking to use the wrong tool.
In less than two minutes, I found these threads have OP of less than 500 characters, and all are of perfectly acceptable quality. Forcing them to place more characters in them to reach a arbitrary limit would probably have lessened their effectiveness, and probably would cause some users to leave, as the more barriers to usage you put into place, the less likely they are to stay, which will mean fewer posts, which means lower SEO, which means it’s all for naught.
(Number in parenthesis are the number of characters in the post).
https://meta.discourse.org/t/signup-is-not-allowed-from-this-account-errors-sporadically/22819/4 (336)
https://meta.discourse.org/t/what-is-the-most-awesome-plugin-for-discourse-that-does-not-yet-exist/31 (146)
HTML Customizations mistakenly applied to admin views? (333)
Grant a badge for all members of a group (467)
How to select a lot of topics fast? (288)