Thanks for the idea, definitely worth trying, but I just did it and no difference, cranks but doesn't start. I was hoping that was it, this not knowing where to look drives me crazy.

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2001 X5 3.0i Old Gold -built and bought 08/2000, now at 220K miles
2005 X5 3.0i Silver Bullet -built 05/2005, bought 03/2021 at 95K miles