So after uninstalling the secondary and cleaning up SQL etc I installed a DP role on it. Once done I started injecting the prestaged content, however I started seeing the following errors in the DP's smsdpprov.log.
[D28][Thu 01/10/2019 04:06:09]:RegQueryValueExW failed for Software\Microsoft\SMS\DP, PxeInstalled
[D28][Thu 01/10/2019 04:06:09]:RegReadDWord failed; 0x80070002
[D28][Thu 01/10/2019 04:06:09]:PXE provider is not installed.
PXE was not flagged during install so after double-checking settings I ran off to Google to see what others have around this. All I could find was people saying to live with it. Seemed strange as this was not on other DPs I checked, so I thought I'd try a quick PXE enable then disable on the DP. By quick I mean enable it, come back later and disable once I validated it was installed via smsdpprov.log and distmgr.log. Sure enough, smsdpprov.log only shows 'PXE provider is not installed' occasionally so the problem is fixed.
Now if it will finish hashing the prestaged content so I can update its boundry group so it will serve its purpose and move onto the next one!