The suffragist characters of the musical “Suffs,” which opened Thursday night at the Music Box Theatre, rarely pause to celebrate their victories. As soon as they’ve achieved something monumental, such as securing a meeting with President Woodrow Wilson or getting the right to vote, another lofty goal appears on the horizon. Or there’s a dispiriting setback. They’re never done. Much the same could be said about the show, itself, written by and starring Shaina Taub. Even after a 2021 run at the Public Theater, which garnered less-than-enthusiastic reviews, and a later workshop to reshape it, “Suffs” still feels frustratingly unfinished.