Hi Bas (and list),
(I'm interested in this topic given that my research group recently started using Zulip to replace the corona-forbidden coffee room. Keeping the list in cc: in case others would be as well.)
I see that you are using Zulip's centralized hosting rather than self-hosting. The free plan comes with a hard limit on the retained message history; that was no problem for our virtual coffee room, but it would be a shame for valuable HoTT discussions. Are you using the free plan? If not, are you just paying for the community out of pocket? (Zulip advertises that they offer discounts for academic users.)