I'm in the same boat with my project to track Twitch chat badges - https://badgebase.co/

I've been observing for a long time how developers promote their side-hustle projects.

Like in the answers here, the most effective approach seems to be talking about your project wherever it fits - HN, Reddit, X/Twitter, YouTube, personal blog, external blogs. The more, the better. The best platform varies for everyone. So just share your project as much as you possibly can.