How do I get better at diagnosing complex EV battery failures?

You have come far. Most jobs are within your reach now. What remains are the tangled failures deep in the battery and the electronics, and those stump nearly everyone who works on them. You are not behind. You are at the edge of the hard stuff. These failures yield to method, not to nerve. Slow down. Trace the system in order. Rule things out one by one instead of hunting for a lucky guess. The techs who solve these are not smarter than you. They are more patient and more organized. Keep a simple record of the tough ones you crack and how you did it. Over time that becomes your own map of the hard cases. The mystery shrinks with every one you write down.