This is what happens with I mount the Win10 Ent. 1809 ISO and run setup.
Your screenshot indicates you are attempting to install a different edition of Windows then you currently have installed. This means if you have Windows 10 Enterprise installed, you must use a Windows 10 Enterprise 1809 ISO, and it cannot be the trial ISO.
Is there a KB I can download and manually install?
No
Apparently there's an issue with Win10 1809 that breaks Intel VT-d (or causes an incompatibility, rather) which 100% a show-stopper for me.
VT-d isn’t required to use VMWare Workstation, which is the reason, you would need VT-x to be enabled.
Even if you actually need it enabled, all you would need to do is disable it, until the upgrade to Windows 10 1809 is complete.
So apparently I am stuck with Win10 1803 for the foreseeable future
You need to download the correct ISO, for the edition of Windows 10, you currently have installed. Once you have an ISO, mount it within Windows, and launch the installer from within Windows.
Here are a bunch of sources for that ISO: Where can I get a clean ISO of the Windows 10 Creators Update (Build 15063), FCU (Build 16299.15), April 2018 Update (Build 17134.1)?
Apparently there's an issue with Win10 1809 that breaks Intel VT-d (or causes an incompatibility, rather) which 100% a show-stopper for me.
I was able to find a single source, that indicated this might be true, but whoever submitted it wasn’t able to communicate very well. It really was complete nonsense, I can absolutely confirm that, Windows 10 Version 1809 does NOT have, an issue with VT-d. This single source, isn’t even worth quoting, because it made absolutely no sense.
As for your latest error.
Win10 update error 0x800F081F - 0x200004
The installation failed in the SAFE_OS phase with an error during INSTALL_UPDATES operation
it’s caused by Developer Mode, you need to temporarily disable it.
The error 0x800F081F-0x20003 in Windows 10 October 2018 update, appears in systems with the "Developer Mode" enabled.
In order to disable it you must do the following:
Go to Settings image -> Update and Security.
Choose For Developers at the left pane and then check the Microsoft Store apps option on the right pane to disable the Developer Mode.
Reboot your machine
Go to Settings image -> Apps.
In Apps & features settings, click Manage Optional Features
Click the Windows Developer Mode and click Uninstall.
Reboot your machine
At this point you will be able to install 1809 without a problem
Source: Windows 10 October 2018 v1809 Update Installation Failed Error 0x800F081F-0x20003
Best Answer
The first step is find out which update is causing the problem.
Go to Control Panel > Automatic Updates and select the option "Download updates for me, but let me choose when to install them". Then when the update is initiated you'll get a dialog asking if you want "Express" or "Custom".
Choose "Custom" and then uncheck all but one of the updates (make a note of which one you leave) and then allow the update to proceed. If your internet connection remains working then proceed to the next one in the list. If your internet connection fails you've found the culprit.
Apply all the updates apart from this one. Then try and find out why this update is causing the problem.