How do you decide whether to keep building or start marketing an idea? (I will not promote)

Background Story: I’ve been working on a website/app/tool (I never know what to call these anymore), to help with hashtag creation. I originally built it because I’m terrible at coming up with hashtags myself, and figured other people might have the same problem. So I made the site and a browser extension. Since AI can generate hashtags now, I added some extra features to make it actually useful, and I have plenty more ideas I want to build out.

Here’s my question: if the end goal is to get people using it and eventually make some money from it, should I spend more time building new features or start focusing on marketing what already exists? If the answer is “start marketing,” what would you focus on first to gauge whether people are actually interested?

Thanks in advance. I'd really appreciate any perspective on this.