Interesting. I have an account with 19k followers, also with low reach & engagement. All real human non paid for followers.
When it was under 10k engagement was high. At this time, there wasn't any insights, so I assume posts had high reach as well. I looked around at other accounts at the time and noticed that when your account is very small you get lots of engagement (and probably exposure to get that engagement), and as the account grows in size this drops off. That is the pattern I observed a few years back, and I put it down to the fact that people have a different relationship with a 'friend' or even 'small business', but feel less comfortable commenting on something more publicly.
Since then I left the account inactive for a year, and Facebook bought instagram and started messing around. I now have 19k followers, but I also have a reach of about 3-4k per post, and of that about 3-400 likes and a few comments. I'm not using hashtags (which this post has reminded me that I should), and if lack of engagement is keeping me in a loop of low reaching posts than I should probably try out those engagement groups or at least do some posts which encourage commenting to boost this.
Interesting idea to start a secondary account as a back up plan....
I wonder how much of this is due to the previous inactivity of the account, how much is due to Facebook buying IG and wanting to push ads, how much of it is a cycle of low reach = low engagement = low reach, and how much is down to the pattern of drop of engagement that occurs as accounts grow.... Any ideas?