You are correct that work can be done without being a billionaire. However, to accomplish much I imagine it would have to become a full time job, at least for some people. Most of us can’t leave our jobs. And if it was going to be a part time endeavor, we might get more results joining an organization that already exists.
I haven’t kept up with the unitarians, but using them as an example you could probably work with them to accomplish some of these goals. You wouldn’t have to build everything from scratch. But you’d have to deal with the existing brand and legacy, so that’s a mixed bag.
Also, I would be hesitant to get too in bed with corporate interests. That could easily lead to enshittification, corruption of ideals, or just losing people because it feels corporate instead of spiritual. But I’m not experienced in running this sort of thing, so this is mostly guessing.