I notice many saas hackers use regular domains for the landing page. Once the user is logged in, they are directed for subdomain. Google uses this approach for all their business suites (ex: https://doc.google.com). But there are exceptions like Netflix who keeps the domain for authenticated/unauthenticated the same.
Using the static landing page that takes up the whole root domain seems like a waste to me. And you'll need to still check the authentication in the backend even for the landing page so you can redirect the users appropriately. So you cant just serve the static html at the end of the day even for the root.
Here is a rough pro/con I can think using subdomain approach.
Do you have your preferred approach?