Keycloak, most likely. We have experience with it.

Fastmail’s calendar invitation support is solid in my experience. And they support access via standard protocols. That is very quickly disappearing with Microsoft 365.

We never used Teams. Once we had the option to switch to non-Teams 365 licenses we did that almost immediately. We used to use Slack, but saw the writing on the wall when Salesforce acquired it, moved to Mattermost in Kubernetes, then Mattermost Cloud’s paid tier when it launched. When Mattermost evicted non-enterprise licenses from their cloud service we never replaced it. Turns out, we don’t really need an internal chat/conferencing app. We’re a few guys within a half hour of each other. If we need to, we hop on a call, send an email, or send a text to schedule a in-person meeting.