The volunteer program initially struggled to attract participants in its first year. ______, word of mouth from early volunteers helped…

The passage describes an initial struggle followed by an eventual, contrasting success. Nevertheless signals exactly that kind of contrast: despite the rough start, growth occurred anyway. Otherwise would suggest an alternative outcome that didn't happen, which doesn't fit a sentence reporting what actually did happen. That is signals a restatement or clarification of the previous point, not a change in direction. Regardless would suggest the growth happened without any connection to the struggle, but the passage frames the two as connected through word of mouth from those who stuck with the program.