I went down a massive rabbit hole for weak starts and no-starts with my (new to me) 2002 Boxster S. It turns out I had a ten year old battery and a new one fixed me right up.
Do you have another car with a new-ish battery in it? Try putting that one in your Boxster and see how it does. The battery try can accommodate several different sizes so you might even be able to get the hold down bolt in there and drive around with it. That's what I did on mine. I pulled a battery i bought back in April out of my SUV and dropped it in my Boxster. It fit and I was able to bolt it down.
This thread was a big help for me:
https://986forum.com/forums/boxster-general-discussions/49850-intermittent-start-problem.html
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