One way to go about this, is to make a script using Python.
One of the libraries that you could use, is Selenium. It allows you to click on web elements, fill, or extract data. You're able to extract followers/following number. And you're able to click on the follow button. Moreover, it allows to load dynamic pages, such as Instagram ones
The only problem to solve here, is to figure out a way of constantly scrolling through the list of that Account.
If the latter is figured out, then all you got to do, is for each account, get followers/following, compare, and if it suits the conditions, register the username into a file text. Once you finish. You could periodically follow these accounts registered in the file, in order to avoid a block.
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I suppose you have previous experience with programming.