Isolated nodes / Runner and external variables

We want to use isolated nodes/runners for customer sites. And have a generic github repo for the global management jobs as a project. This works ahead multiple inventories. But how can I access customer specific variables. In the old setup we had a awx instance for each customer so a own github repo with a var file. Is there a possibility to read a externa var file. Or do I have to use a vault such azure key vault or hasicorp