For the Flow Instance History reports to use Names for assignees and users who have completed the steps, 2 fields need to be added to Flow Instance History object:

Assigned_To_Name__c (text 255)

Completed_By_Name__c (text 255)

Once added, 2 Apex files need to be added to the Org: an Apex class and a trigger on Flow Instance History object. They are attached below.