My email list hit 5,000 subs last Tuesday and I almost missed it

I was cleaning up my Mailchimp account last Tuesday when I saw the number was 5,012. I started this list two years ago with 30 people from a Facebook group in Tampa, mostly friends and family who signed up to be nice. What surprised me is that the last 1,000 came in just 4 months, all from one lead magnet I threw together in an afternoon. It was a simple PDF checklist for small business owners, nothing fancy, and I did not even promote it that hard. The weird part is my open rate dropped from 38% to 29% at the same time, so more people does not always mean better engagement. I am trying to figure out if I should clean the list or just let it ride. Has anyone else seen opens tank right after a growth spurt like this?