I've maintained a fairly popular open source project for over 13 years[1]. The software is basically "complete." How does funding work for someone like me? I have no initiatives with it that require funding. Occasionally, I need to fix a bizarre obscure bug, or support a new python version/feature (async/await being the last big one). But otherwise, I just field questions a few times a month. Truth be told, I'd rathe…
If you believe you still get an ongoing benefit (reputational or whatever) by being its maintainer and continuing to support it, and if you still want that benefit, then you have to keep up the work. Very little in life is truly free, unfortunately.
You could also see if there's a way to monetize your work on it. If people want support or need bugs fixed, they can pay you to do so, for example. Might even look into GitHub Sponsors, if you haven't already. I do get that it's harder to solicit donations for something that's "finished", though.