I wonder if it would help to have reserve selections so that if someone's circumstances change after being picked they have the option of dropping out without leaving the rest of those selected a teammate down? When I signed up for the mission I was certain that I'd be available, but afterwards I ended getting surprise extra hours at work and an illness that lasted weeks at the same time, and while I participated, I ended up hampering the team's pace.

Having people there who are explicitly willing to step in short notice might make things run smoother, especially if players can quietly swap out without being penalised before the mission is underway. Usually people sign up in good faith, but shit happens sometimes.