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The site is not ready but the domain is owned. Would it be better to have just a 404 or a coming soon page? For SEO purposes.

A 404 simply creates an SEO vacuum.

Set up a keyword-rich "coming soon" page and provide a form so visitors can leave their email addresses. When the site rolls out, shoot them all an email to let them know. Even better, give them some premium access, a sneak peek or some other promotional offer. No reason you can't start generating interest now.