Slack has raised our charges by $195k per year

We're using teams in my new company, which is awful for textual communication (lacks threads in chats, groups are more like old forums than new IM). I've been experimenting with self-hosted Mattermost but it seems that it also requires paid license in some situations (e.g. does not have groups for some reason in the free version).

I was unable to find another system. Would anyone recommend me something?

I was considering moving from Slack (free version) to Teams (paid) for a new project starting in October because my workplace already have a license for that. Seems like it will have less features but no 90 day retention annoyances.

You seem to have some experience with both, do you think I am making a bad decision for a ~30 person team?

Others suggested Matrix, but I have a feeling they are implicitly assuming self-hosting. I do think Element works quite well, but I have only used it personally with matrix.org for basic chat, never for work. It does work on both Android and iOS as well as Linux, which is why I use it.

I use Element in an organisation of around 300 people, most of which are non technical. 98 % of them really dislike Element and I really understand why. Even for the most technical people it just does not work reliably like WhatsApp, Telegram or iMessage, which are some apps those people use privately. I really hoped that it'll all get better with Element X, both on Android and iOS, but it's not. I wouldn't recommend it to anyone really.