When projects want to formalize higher level governance - "Who decides who decides?" - one common path is to join one of the existing non-profit Foundations that host community-led open source projects. Foundations often provide many useful services, like build infrastructure, legal support, community mentoring, or fundraising assistance. But the most important service Foundations can provide are a strong framework for constitutional choice governance. Project leaders can define specifically who their project management committee is, and then work with the Foundation's governance to maintain that.
The top Foundations each have very different styles of formal governance. Choose A Foundation provides a guide for understanding the ethos and governance style of several Foundations, like the ASF, Linux Foundation, Software Freedom Conservancy, or more.