There isn't a way to post to Nostr and force other clients to not render the post. Until that type of functionality can be agreed on, your long-form posts will show up with the rest of your posts in your profile. You could make a new nsec that is only used for long-form posts.

I played with habla.news and yakihonne.com today, and I really like yakihonne. Amethyst even displayed the formatting correctly.