I mean knowing average company security, they probably are part of a botnet.

I would honestly expect that with modern spam filters the random rate fluctuations of spam getting through the filters would have a bigger impact on the spam you see in your spam folder than the actual number of devices sending it.

Also, if your theory were correct that those make up a sigificant percentage of spam sending devices I would expect home office trends during the pandemic (which often use devices sending all their outgoing traffic over VPNs with relatively strict firewalls) and especially complete closures at the start of the pandemic to have a big impact on spam volume which has never been brought up anywhere to my knowledge.

I can't remember the last time I got a spam email honestly. I changed my address like 5+ years ago and it just stopped and never resumed. It's not the email provider filtering them either, the only thing it catches are legitimate but automated messages.

Email open rates on the weekend are way way lower than during the week.

In an afternoon you can clean up your mail, cancel subscriptions, setup some anonaddy or simplelogin emails for future control, and in the span of a week, you kill the remaining spam emails that come and you'll be mostly free of spam forever (aside from very sporadic junk that will be easily deleted).