The sub is r/destroymyapp
In my opinion, the best way for someone to make their app the best it can be is to give hard feedback. Tell me what sucks and why rather than sugar-coating it. The goal of r/destroymyapp is to provide a place for you to receive that feedback on your app, even if it's harsh.
Getting feedback from your friends and family often sucks, they will tell you what's good rather than what's wrong, which makes it hard to fix potential faults that could affect the performance of your app on release.
I've seen similar subreddits for getting feedback on games such as r/DestroyMyGame, and something similar for software design would be helpful. I know I would use it.
Come by, leave some feedback or post some indev/completed screenshots of your app and get some criticism.
Don't take anything personally. This sub wants to help make your app the best it can be.