In the end, we picked Signal. We liked its open-by-design approach, its serious and ongoing privacy and security stance and having a Signal app on our GNOME (and Windows) desktops. It performs very well on our Android handsets and our desktops. Moreover, it wasn't a big surprise to our small user community; it feels much more like WhatsApp than, for example, Riot.im, which we also tried extensively. Having said that, if we were trying to replace Slack, we'd probably move to Riot.im.