Do you have a version of Google Workspace that includes the investigation tool?
I was just able to create a simple search of an "All staff" email I sent out using the exact subject line and the "label is inbox" conditions.
To me, it looks like a message is read if the label does not have the "unread" label associated with it.
Run the above query without the "label" condition and you'll see label statuses of "inbox" or "inbox, unread".
From here, it is easy to download the results to a Google sheet.