The highest-impact growth marketing tactics for a B2B SaaS startup with zero customers is often direct outreach. When you're at a point where you don't have customer validation, it's incredibly hard to get useful data from very small amounts of traffic. So in a zero customer state, your highest-impact effort is direct outreach.
Signing-up customers one-by-one is the best way to get customer validation and you'll learn a ton of valuable data along the way.
Once your customer validation is complete and you have at least a handful of customers *actively* using the product, then you have the rationale to start spending and experimenting with growth marketing tactics.
Happy to discuss the customer development process with you and share with you what has been successful for me in doing direct-outreach to enterprise customers.