Drivers not working on Windows 10 installed (CLI process) on external SSD running on Mac mini - EFI Started But Screen Black After Win Logo
I was able to recreate the problem shown in the image you posted. Apparently, at least the driver for the internal drive has to be loaded from the WINSTALL volume before the dism command will work properly. I update the instructions so you will proceed with the GUI steps for installation until the window below appears.



At this point, the drivers have loaded, so you can proceed and open a Command Prompt window and start entering commands.

I changed the examples in my answer to be from an actually installation of Windows 10 (1909) on to a external HDD connected to a 2018 Mac mini. I also changes some of the steps, so you should thoroughly review the answer before proceeding.

Let me know, if you have any other problems.

Update 1

The image you posted was a bit blurry. A far as I can see, you did type the following command in correctly. 

chflags nohidden /Volumes/WINSTALL/label/.d*


However, I believe you had pathname expansion disabled. This would cause the error message shown in the image. I replaced the command with two commands, that do not require pathname expansion.

I should note that according to the history, you entered the command give below, which should have produced an error message.

bless --folder/Volumes/WINSTALL/label --label "Windows"


You should have typed the command given below. 

bless --folder /Volumes/WINSTALL/label --label "Windows"


This command would have produced the following files.

/Volumes/WINSTALL/label/.disk_label
/Volumes/WINSTALL/label/.disk_label_2x


If these files do not exist, then the new commands given below would fail.

chflags nohidden /Volumes/WINSTALL/label/.disk_label
chflags nohidden /Volumes/WINSTALL/label/.disk_label_2x


Update 2

Your "Update 2" given no indication that a failure occurred. In other words, the image shows that the command appeared to work correctly. However, since the chflags command is causing problems for you, I decided to remove the the use of the chflags entirely from my answer. Instead, I added the following Windows command to step 8.

attrib -h t:\label\*


Update 3

The command attrib is a part of Windows. The command works when entered in a Microsoft Command Prompt window. The command will not work in a Apple macOS Terminal application window. You do not have to enter the attrib command until step 8. During step 8, you will have a Command Prompt window open. 

Update 4

To open a Command Prompt window, you are suppose to press the shift and F10 keys at the same time.

You copied the WindowsSupport folder to the WINSTALL volume. The instructions in step 4 stated to copy the contents of the WindowsSupport folder to the WINSTALL volume. The $WinPEDriver$ and BootCamp folder should appear in the root folder of the WINSTALL volume, as shown below.



Update 5

I am glad to see you have successfully installed Windows 10. If you feel my answer solved your problem, then I would ask you to accept my answer. This will place a green checkmark next to this answer. This helps other users identify that a solution was found.