Thanks for your posting and sorry for the late response. I wasn’t notified about the new topic for some reason (I will look into it). I would also recommend what Knaledge suggested: using an external tool to manage tasks such as Trello or Asana (which we currently use here at Gurock Software and are quite satisfied). You can also integrate TestRail with other tools via the defect/reference urls or even write a custom defect plugin for a more advanced integration:
For todos/tasks in TestRail, we usually recommend using the Todo page that was specifically designed to quickly process open tests with filters and support for adding test results directly on this page.
I hope this and just let me know in case you have any further questions.