Since you can’t modify unattend.xml or really do anything else to keep it enabled through sysprep (which auto disables it)
Create SetupComplete.cmd (make sure no hidden extensions like SetupComplete.cmd.txt)
Edit that file, put this in there:
net user administrator /active:yes
(optional change password) net user administrator new_password
After Windows is installed, but before the logon screen appears, Windows Setup searches for the SetupComplete.cmd file in the %WINDIR%\Setup\Scripts\ directory. If a SetupComplete.cmd file is found, the file is executed. Otherwise, installation continues normally. Windows Setup logs the action in the Setupact.log file.
That should do it, now that Template should keep local admin enabled even after sysprep.