I had to abandon the "concurrency-safe local repository" because it's not currently safe for concurrency:
The standard repository should be safe for multi-threaded use, with the option -Daether.connector.resumeDownloads=false in .mvn/maven.config.
But I did find the OkHttp extension for faster dependency downloads, plus I threw in a profile for frontend which can skip npm install.
It looks like the Maven project type in Jenkins doesn't support core extensions (https://issues.jenkins-ci.org/browse/JENKINS-30058), so I don't think the main Maven jobs are getting the benefit of Takari Smart Builder or OkHttp Aether Connector. Consequently, these changes will just speed up local builds for now.
We should see more benefits from this when we start using Pipeline builds: ZNTA-1414.
I don't anticipate any problems with parallel building or the smart builder, but if we need to, we can use the mvn options -T 1 or --builder singlethreaded to deactivate it.
From a quick glance, it looks like our Maven plugins should all be parallel safe: