Can't get Templates to work

I think this is more of a problem with documentation or my stupidity, and I apologize if this has been answered before, (but my search didn't reveal an easy answer) but I can't get Templates noticed with my AspNetCore project

I installed OpenApi-generator with NPM and I can see the openapi-generator jar file in C:\Users\robera22\AppData\Roaming\npm\node_modules@openapitools\openapi-generator-cli\bin.

If I make changes to this (WinZip, don't unpack, change e.g. aspnetcore/3.0/Controller.mustache, save mustache, save zip) then generation works with my command line and I can see the changes I have made.

openapi-generator generate ^
-i Planning\OpenApi\ToolboxV3\MmtBatchWizard.v1.yaml ^
-g aspnetcore ^
-o Servers\v30Local4 ^
-t Servers\Templates ^
--additional-properties=^
aspnetCoreVersion=3.0,^
operationIsAsync=true,^
packageName=Medtronic.Toolbox.MmtBatchWizard,^
packageTitle=MedtronicToolboxSandbox,^
packageVersion=3.0

However this is horribly cumbersome, and takes forever to re-jar the large openapi-generator file

The documentation suggests that I should be able to specify single templates so I set up a Template folder.

First I placed a single amended Mustache File in there (Controller.mustache), no effect
Then I copied the whole aspnetcore/3.0 folder there
Then the aspnetcore
Then the openapi-generator folder
Then the openapi-generator.jar file

None of these had any effect. Still picking up the amendments in the npm/@openapitools

What am I doing wrong in what should be a very simple use case

Start with the documented -t option and the command shown in the issue, then trace how the npm-installed generator locates aspnetcore templates. Compare that behavior with the amended Controller.mustache and the template directory layouts the reporter tried. Done means documenting or correcting the lookup behavior so a single customized template is used without re-jarring the generator.