The Kunena team has announce the arrival of Kunena 5.1.18 [K 5.1.18] which is now available for download as a native Joomla extension for J! 3.9.x. This version addresses most of the issues that were discovered in K 5.1 and issues discovered during the development stages of K 5.1.
Select in the Kunena configuration -> (tab) Users -> Display User Name = No.
In addition you can also choose in the Community Builder "Configuration -> General -> Name Format = Name" (this setting is for the CB profile).