I've said it before (so at the risk of annoying people ;-) I can highly recommend Zulip. We briefly used Mattermost (couple of months) before realising the Zuli

I've said it before (so at the risk of annoying people ;-) I can highly recommend Zulip. We briefly used Mattermost (couple of months) before realising the Zulip threading model was a way better fit for us then what Mattermost offered. It might feel a bit odd in the beginning but after you've become accustomed with it you'll love it!