My Most Successful Project Doesn't Make Any Money

I'm fullstack developer that currently specializes in Web3/Ethereum for my day job. In my spare time, I build side projects that I get spontaneously get ideas for. Most never take off, let alone earn me any money.

Over a year ago, I built a developer tool (https://louper.dev) for smart contract developers that build contracts using a specific design pattern. It's my most successful project to date in terms of active users.

Right now the app is free to use but I can't help feeling I could turn it into something that could provide a nice side or even full-time income. How would I go about doing this?

I feel like anything I charge for would have to be a value add to what I currently have. Right now the app serves as more of a public good but I think maybe adding some compelling "Pro" features might be the way to go.

Does anyone else have experience turning a free product into a paid product without alienating their userbase?