2 days ago I subscribed to GitHub sponsors, and I already got my first sponsor!
Following the example set by Caleb Porzio (who's making more than u$d 100k/y doing open source), I have decided to use the sponsorware model to fund my project. It works like this:
In a few months, I will also start creating instructional videos, explaining how to make the most out of the plugin. According to Caleb, this is the biggest money-making strategy.
I have also decided to add a middle tier (at u$d 70/m), where I provide Slack-based personal support, to help users of my plugin set-up GraphQL with WordPress, troubleshooting, and answering their questions. A user needed help to develop a functionality, so he decided to sponsor me <= my first sponsor ❤️
Finally, I added a higher tier (at u$d 700) for corporate sponsors. I plan to ask around in the WordPress community if their companies may be interested in participating. That would be a win-win: They get plenty of face from contributing to open source, and I get the certainty that I can make a living wage from my work and can focus on the development of the plugin (and not on marketing, which is not my forte).
This first-sponsor milestone is extremely significant for me: I've been working on my code for a few years, and it's the first time I can make a bit of money from it. (Until today, my highest source of income was writing articles for online magazines, using the ideas and material from my open source work.)
Cheers!