I’m a fan of using ads to accelerate growth, but you don’t have to. I bet most profitable side projects got there without ads. And you probably shouldn’t spend money on ads in the early days, for a bunch of reasons. In general, I’d focus on organic growth channels to start with and only switch to ads when you have that working, you have revenue that’s growing, churn isn’t too high, etc.

Ads are just conversations at scale.

Elon Musk has conversations at scale on Twitter, no ads needed.

If you know who would use your product, find them and have conversations. Talking to real people is the first step to great ads anyway.

If you don't want to have conversations with your users, or don't know who they would be, maybe you would be better off building something else.

You can get traction on projects without running ads. Show your product off to relevant communities of people asking for feedback, etc. If people start buying, start considering ads.