As I've said before, Matrix really is the only viable open source solution for in-company communication.

Every other solution (Zulip / Mattermost / whatever) is too risky, they could easily bait-and-switch you like Gitlab did, by moving important features to different tiers, or engage in other shenanigans afforded by the open core model.

Matrix has a bad reputation because it used to be downright terrible (first time I tried it, in like 2018-2019), but is a lot better now.