I did this for 10 years, 2 years as a freelancer (barely getting by) and 8 years as an employee. The project has now completed its lifecycle.
You don't have to be a founder, you just need to be an expert. In terms of career development, I think the ROI is better on joining an existing project. But "progress depends on the unreasonable" and some people become founders nevertheless.
I did this -- working on open source founded by other people, from Bugilla to GNOME -- for two decades, and my experience is similar. Contract work is available for customizing open source broadly, and pays well, but it has all the common downsides of contract work. Notably, it conflicts with the growth-driven VC model which is where open source has ironically become a cornerstone.
You don't have to be a founder, you just need to be an expert. In terms of career development, I think the ROI is better on joining an existing project. But "progress depends on the unreasonable" and some people become founders nevertheless.