Ask HN: How would you monetize a reasonably popular free service?

I built and run Reqres - A hosted REST-API ready to respond to your AJAX requests (https://reqres.in/) - a free tool for developers. It receives millions of requests a month (this is the devs using the tool), and the homepage itself gets 20-30k unique visitors a month.

I run ads on the homepage which generates $200-300/month.

I'm looking to add a Pro tier to the service to start generating monthly income (through subscriptions) from it. I have a mailing list of ~200 people who clicked "I'm interested in the Pro tier" button, and sent them a survey to try and dig into exactly what devs would want from a Pro tier. I got 4 responses back.

What would you do to find out what people want from a Pro tier? Would you even add a Pro tier? How would you monetize this free service?

Would love to hear your thoughts: hello@benhowdle.im

(I have this PDF linked to on the homepage: https://www.dropbox.com/s/2w7gaeydx02qs40/Reqres_Advertising_Proposal.pdf?dl=0, which I had a company trial it for one month, but decided the fee wasn't worth the CPA they got from clickthroughs)