I want to understand the level of activity across the complete email marketing base and gauge email marketing engagement.
The way I want to define "engagement" is customers who engaged in the month (Open Reach) = # of customers who opened 1 email in the last 6 months.
How do I create this report?
An open of an email is only accounting for someone having the images downloaded in the email. They may have looked at it because of the subject line. But truthfully, I wouldn't consider them to be engaged. People that are engaged are going to visit your website, fill out forms and click on the CTA in the email (which also has bots but if you have it go to a landing page you can at least look at that report to get a more true picture), registering for webinars, going to events, etc.
In any case, if you want to know how many people have opened an email in the last 6 months you'll need to create a smart list with a filter than they have opened an email and use a constraint for in the past 6 months. That will give you the list and number.
I hope this helps!