Nonprofits don't have to do anything charitable. There are lots of different kinds of nonprofits; not all of them do charitable works. I doubt that the time slips that JWs turn in has to do with maintaining their nonprofit status.
501(c)(3) status is "exclusively for religious, charitable, scientific, literary, or
educational purposes, or for testing for public safety, or to foster
national or international amateur sports competition, or for the
prevention of cruelty to children or animals." Lots of churches and other religious organizations without evangelizing or proselytizing as a primary activity have 501(c)(3) status. I have some relatives who started their own church and they have 501(c)(3) status. They do the kind of normal outreach that churches do for their own members, but they're definitely not a charitable organization.
Here's a taxonomy of nonprofit organizations: https://learn.guidestar.org/help/ntee-codes.