I had a weak start/no start on mine and it turned out the battery was 10 years old even though it looked new.
The ground strap can be a common issue if it gets corroded. I think there is one by the battery and also one on the engine. I know the immobilizer gives people fits too. Sounds like you gave that a look already though.
these are the threads i saved on troubleshooting:
https://986forum.com/forums/boxster-general-discussions/49850-intermittent-start-problem.html
https://www.pelicanparts.com/techarticles/Boxster_Tech/81-ELEC-Battery/81-ELEC-Battery.htm
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