Might be worth creating a custom HubSpot score/health field of your own to really map out what you want an engagement score to look like. You could leverage a lot of the last email click/open/etc date fields in those to help come up with a value to track these items. What I like is you can also build positives and negatives into these for say haven’t clicked an email in X number of days. Have you messed around building a score field before in HS?
If you are doing list management side you can use active/smart lists to use a lot of the same data to pull in those individuals into certain campaigns. I do this heavily in our world to manage who will and wont be getting emails and to make my life a lot easier.