Whenever I get 10 new sponsors (at u$d 14/m), I start developing the most up-voted feature from a special "Sponsorware" list (which I'm completing as I'm writing this milestone)
Once implemented, the new feature becomes available to all the sponsors, via a private GitHub repo
As soon as I get 50 new sponsors, the new feature becomes open source, accessible to everyone via the public GitHub repo, and is integrated into the plugin
In a few months, I will also start creating instructional videos, explaining how to make the most out of the plugin. According to Caleb, this is the biggest money-making strategy.
I have also decided to add a middle tier (at u$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).
I hope the sponsorware model works, and I can make a living while working on open source. I'll keep writing updates on how it goes, here on my blog, and on IndieHackers.
Update 15/09: The list of sponsorable features is ready permalink
I have now listed down all the features I plan to implement if I can get the funding. Right now, there are 23 of them (some of them are low-effort, so they can be bundled together):