The small nonprofit had relied entirely on volunteer labor for its first five years. ______, growing demand for its services made hiring…

The passage describes a straightforward progression over time: years of volunteer-only operation, followed by a change necessitated by growth. Eventually correctly signals this temporal progression. Conversely would frame the second sentence as a direct opposite of the first, but hiring staff is a natural continuation of the organization's growth, not a contrasting case. Namely introduces a specific example of something just named, which doesn't fit this shift over time. In other words signals a restatement, but volunteer reliance and hiring paid staff are different stages, not the same idea restated.