efscher

https://z-lib.gs/

I’ve been using Voipo for years, no complaints.

There is zero requirement to use any server or service associated with either New Vector or the Matrix Foundation.

There is also zero requirement to use the server (synapse, dendrite) or client (Element) implementations developed by these guys.

For example, I use conduwuit for my home server, with neochat and fluffychat clients, and they talk using published APIs and nothing else…

In the meantime, Evolution has had EWS support for years… no Rust involved.

Radicale. Not the fastest, but has worked for years.

Orion doesn’t make the list?

WPS Office and FreeOffice. Each with their own caveats, but one just might work for you.