Kinda new to gradle, but im an experienced devops guy, and I never had trouble setting up a local maven mirror and configuring build pipelines to use it, except when I started using gradle. I can see in the documentation clearly how to consume a maven mirror in gradle, but nothing about how to consume a gradle plugin mirror (I can set up the gradle plugin mirror but having a hard time configuring gradle globally to use my sonatype nexus as a gradle plugin mirror).
It is the same as for other repositories, nothing special.
Just that you have to declare it as plugin repository, so within the pluginManagement { repositories { ... } } block of the settings script.
You are in an init script, not in a settings script. pluginManagement { ... } exists on a Settings object that you work with in a settings script.
In an init script you work with a Gradle object.
If you wrap that block in a beforeSettings { ... } to configure the settings script when it is evaluated, it probably will work.