Instagram is not out for you. Your pool of followers obviously are not interested in your content. You need to find the people that do.

Why do you think the followers you have like your content? If you followed/unfollowed, then you got those people based on that strategy. Doesn’t mean they will engage with your content. I only follow/unfollow people I know will have some interest in my page.

For example, one of my clients has a car page. I will follow-unfollow people with car profile pics (not all niches have it this easy, but I’m sure if you thought about an identifying feature of your audience you could come up with a criteria.

If you want high engagement, then you need to identify what your instagram is about, find the appropriate content to post, hashtags to use, and people you should actually follow that you know already like the content you will be posting. Focus on that and measure how your engagement rate changes over time.