Best way to give site sub-page it’s own domain?

You are on the right track.

  1. Point your add-on domains to WordPress root
  2. Create WordPress landing pages
  3. Point to landing page using add-on domain
  4. Rel=canonical will prevent duplicate indexing penalty
  5. All landing page links will automatically point to canonical domain
  6. Naturally you will do all your editing from the canonical domain
  7. If you run into trouble check your .htaccess file for redirects