What I ended up doing was upgrading my MDT to the 6.3.8450.1000 build (as you noted). Then, I tried my non-MDT TS...it still failed. I have no clue why. So I then recreated my entire OSD MDT TS. A few small TS tweaks and it now works with the older and the current Gen5's.
For my 1803 deployment, I have the "apply operating system" ts set to "use an unattended sysprep answer file" and my unattended.xml is this:
**********************************************
<?xml version="1.0" encoding="utf-8"?>
<unattend xmlns="urn:schemas-microsoft-com:unattend">
<settings pass="oobeSystem">
<component name="Microsoft-Windows-Shell-Setup" processorArchitecture="amd64" publicKeyToken="31bf3856ad364e35" language="neutral" versionScope="nonSxS" xmlns:wcm="http://schemas.microsoft.com/WMIConfig/2002/State">
<OOBE>
<HideWirelessSetupInOOBE>true</HideWirelessSetupInOOBE>
<NetworkLocation>Work</NetworkLocation>
<SkipMachineOOBE>true</SkipMachineOOBE>
<SkipUserOOBE>true</SkipUserOOBE>
</OOBE>
</component>
</settings>
</unattend>
**********************************************
Then, at the bottom of the auto created group called "install" (this is where the OS is applied and such). I have a "run command line" that sets the power plan so the computer does not sleep during OSD. This cmd line is:
**********************************************
PowerCfg.exe /s 8c5e7fda-e8bf-4a96-9a85-a6e23a8c635c
**********************************************
I apply drivers next. I integrated the HP MIK into my SCCM and now I create driver packages via that method. Seems to create smaller packages than previous when I was downloading the larger driver packs. I filter each driver pack by wmi query model.
At the bottom of the drivers section, I increase the ccm cache size to 20GB using a powershell script. At the end of the OSD TS, I reset it back to 10GB. This allows larger packages to be installed during OSD.
I hope this helps a bit.