I learned product management skills from writing my unpopular side projects and responding to my users.

If you're doing it even partly for purposes of looking good in an interview, I can assure you that it really doesn't matter. Pretty much anything you do other than just "I wrote this code at company A to do features B,C & D" will immediately set you apart from the crowd.

I've interviewed well over 100 people and I can count on the fingers of one hand the number that even mentioned some kind of side project.