Show HN: Find free Android apps without ads or in-app purchases
reallyfreeapps.appspot.com
reallyfreeapps.appspot.com
If I can't find something ni F-Droid, then I search Google for e.g. "insect id app android site:reddit.com" and read all the comments, and then and only then do I risk installing something.
Based on my experience, I would estimate that more than 99% of apps in the official app store are malware of some kind.