Mattermost isn’t e2ee, but if the server is run by someone competent and they’re allowed to see everything anyway (eg it’s all group chat, and they’re in all the groups) then e2ee isn’t as important as it would be otherwise as it is only protecting against the server being compromised (a scenario which, if you’re using web-based solutions which do have e2ee, also leads to circumvention of it).
If you’re OK with not having e2ee, I would recommend Zulip over Mattermost. Mattermost is nice too though.
edit: oops, i see you also want DMs… Mattermost and Zulip both have them, but without e2ee. 😢
I could write a book about problems with Matrix, but if you want something relatively easy and full featured with (optional, and non-forward-secret) e2ee then it is probably your best bet today.