@HonzaUH
Here I am three week after your post and unable to use your solution. Based on the change in Planner icon, either MS changed something, or I'm using the wrong connector.
There is no Dynamic Content called "Completed by User" or any reasonable facsimile thereof and when I tried entering
triggerBody()?['completedBy']?['user']?['ID']
as manual input, the flow failed at that step, and I received the below error:

Am I missing something? Did MS change the Planner connector in the short time between your post and my attempt to apply your suggestion today?
Any help is greatly appreciated!
Kyle