How do you structure a Saas domain?

Help needed!

I am in the sake of building a saas MVP, the indie way. Since I am just learning to code, I have a lot of questions beginning with how to structure the domain.

I want to use Wordpress to start blogging asap while constructing de MVP in order to get noticed for search engines and get traction as fast as possible. Yes I know Hugo, Jekyll, Gatsby etc... are there but I know Wordpress pretty well, and I have other learning priorities.

I also want to take feedback for potential users while building the product.

Should I make...?

• The home "saas.com" a pure html/css landing page for very fast loading.
• A subfoolder like "saas.com/blog" for a wordpress installation.
• A subdomain "app.saas.com" for the web application itself.

In addition to that, I plan to host the blog in a managed hosting service (too little time and knowledge to run it in a self managed hosting, just for security reasons) and the app perhaps in Heroku at initial stage.

What path do you recommend me to follow? What do you think is best for SEO?

Thanks for your comments and suggestions.