I fail to sign a Windows Audio Driver for a Virtual Device

Question

I have developed a Virtual Audio driver based on https://github.com/microsoft/Windows-driver-samples/blob/main/audio/simpleaudiosample.

So far so good, I was able to test and all. Now came the time when I would like to Release-Sign the driver. I started to dig into the Microsoft documentation starting at https://learn.microsoft.com/en-gb/windows-hardware/drivers/install/driver-signing. After digging in this rabbit hole for weeks and followed for example https://learn.microsoft.com/en-gb/windows-hardware/drivers/install/release-signing-a-driver-package-s-catalog-file and https://learn.microsoft.com/en-gb/windows-hardware/drivers/install/release-signing-a-driver-through-an-embedded-signature I am officially giving up and coming here.

If I install the driver, I get an error 52 with devcon status mydriver. If I use the command Signtool verify /kp /v /c mydriver.cat mydriver.sys then I get:

SignTool Error: Signing Cert does not chain to a Microsoft Root Cert.
SignTool Error: File not valid: .\mydriver.sys

I would love some pointers to help me figure out how I am holding it wrong. (Out certificate is from DigiCert and it is an EV Code Signing one as per https://learn.microsoft.com/en-gb/windows-hardware/drivers/dashboard/code-signing-cert-manage).