Jim,
Your frustration is understandable. Managing very large courses seems to unearth extra surprises.
If you know this already, please bear with me:
Announcements are a good solution--provided that the course intro includes instructions to students to set their notifications to forward announcements to their email of choice. (A default, but students can turn it off on their end.) its all about front loading and preparing the instructions for the course in advance, so the students know how to set-up and what their responsibility is in reading announcements.
Some instructors aren't aware that 1.) announcements remain in the course in the announcement tab, 2.) can be set by the instructor to appear/preview on the homepage to be un-missable, through course settings, 3.) also auto-send as an email to all enrolled students' emails' on record--found in their user "Account" settings in the left side global menu.
Alternatively, you may elect to send yourself an email copy of all your own messages (in these account settings on the left global menu. That way you can check your preferred email and skip the Canvas inbox of 900 sent emails. The inbox is great for keeping FERPA compliant date/time stamped records of student interactions. It clearly falls short on volume courses, but I recommend archiving instead of deleting. Those rare circumstances when records are needed for a legal dispute make it worth keeping these.
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