Completely unrelated : if I want to reference a config value inside one of my lambda, I’m forced to use env variables ?

No - you should be able to just reference it directly.

let config = new pulumi.Config();
let value = config.get("value");
cloud.timer.daily("tick", () => {
  console.log(value);
});

Is that your use case?

I need to look how this is serialized