I've always relied on Google Ads and eventually SEO for my SaaS products. For SEO, I've had good success with having the landing page be an unauthenticated version of the app itself (modified to include SEO friendly text), allowing the users to immediately start using a limited version of the app which eventually prompts for signup. After signup, any data from the landing page shell gets pushed into their account.

This significantly reduces bounce rate compared to a traditional landing page and I've had good success getting to the top of popular search terms after a few months/years.