Is your website seasonality or focused on specific region / location?
I have a site got similar "volume" with your example, it's with 10M impressions/week, 1M clicks/week, and over 1M valid URLs. Last time I remember got the drop was caused by public holidays in specific location / region.
Otherwise, it could be penalty.
You may check: in the dropping days vs. any of common days:
- Is the website covering same distinct number of keywords?
- Are there same distinct number of Page URL got impressions?
- What's their ranking status?
- How about your branded keywords status?
- How about the industrial keywords status?
IMO, point 2 is important, e.g.
If you see usually you have 30K page urls got impression, but in the dropping day, only 20K URLs got impression,
then you can export the urls, find those lost impressions (no impressions in the dropping day), and compare to the target page urls where "spam backlinks" you found, if they got matched, then that's a proof of penalty.