Although I'm not OP, I use Phabricator (https://www.phacility.com/phabricator/) at work and I absolutely love it. It's much better than Github for our use case, as it has subtasks, task merging, priorities, Kanban workboards etc. Tasks can block another task (for instance, the "Implement OAuth2" task can block the "Build a private API" task). You can flag objects for later and set specific privacy rules for every task (for instance, you can hide a security bug from the public until it's fixed). It's all tightly integrated with other Phabricator apps, which is a plus. Also, it's open source.