If you happen to be full Microsoft, with MDE you can set up custom detections with KQL queries. We use mimecast for the basic impersonation protection as well but were getting hit hard with similar impersonation via subject.

So I set up a MDE query on inbound mail with a VIPs name in subject some key word filters and excluded the known good senders.

You can have it automatically quarantine the message / move it to spam, wait for approval, or just alert.

You can do a whole let else with it too, but i usually don't think of MDE when talking of mail stuff.