I can work out what moved, but I need the raw counts and the details of the domain change first. Right now there are two possible causes, and the data you sent can't tell them apart.
What I need (required): 1. Per send counts, before and after the drop. For each campaign from about 4 weeks before the drop until now: date, segment, delivered, unique opens, unique clicks, bounces, spam complaints and unsubscribes. I recompute every rate from counts. "About 45% to 30%" and "clicks about the same, I think" are too rough to separate causes. If your ESP shows Apple Mail Privacy Protection opens separately, include them, because machine opens can move the open rate on their own. 2. When the drop started. "This month" isn't enough. Which send was the first low one? 3. What IT changed on the domain, and when. Please ask for the exact change and date. For example: new sending domain or subdomain, SPF, DKIM or DMARC records, a custom tracking domain, or a link branding change. A tracking change can make opens fall while clicks hold. An authentication change can hurt inbox placement. They point to different fixes.
Optional, with the defaults I'll use: - Gmail spam rate: if you have Google Postmaster Tools, send the spam rate by date. Default: complaints from your ESP counts only, labeled as a different measure from Postmaster's. - Other changes: template, from name, segment rules or send time changes. Default: none assumed.
When I have these, I'll return the send table, before and after rates built from summed counts, the hypotheses ranked by how many rows support them, and the check that would confirm each. I won't change any DNS record, ESP setting or segment without your approval.