Update:
I updated the Job code, hardcoding JSON Service Account credentials to verify that what works locally isn't working in GKE (it still didn't work in GKE).
I then stripped back the cluster configuration to the bare essentials and removed the cluster network / subnet setup.
Once I did this, the job connected to CloudSQL 🙂
So, I guess I'll have to do some reading up on networks / cluster configs. For now I'm happy that I can talk to the DB!
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