Nice work here!
I had made it work with a manual workaround when discussing with @kolaente how Vikunja could improve the logic of projects/sub-project/tasks/subtasks to make it more consistent and functional (which would as a result resolve the feature request here). I think the project would greatly benefit from this approach as it would address many other requests and use cases.
See screenshot here on how all tasks of a parent project are displayed: Improvement suggestions - #4 by avatar1024
See the logic explained here: Improvement suggestions - #6 by avatar1024