PM - it does have intervals that are prescribed by the manufacturer. Doing anything too early is a total waste of time and energy in my book. Unless you find online clear evidence that the PM schedule indicated by the manufacturer is not working out. I do PM on schedule though and only stretch it on items that are clearly not a safety or reliability issue, for example a cabin air filter ...
So, I think your plug and coils were unnecessary. Newer boxster is a 60k spark plug change interval. The squeaks I would have lived with a couple years more, its not that they were a safety issue. Then you may need new struts anyway and do it all at once. And I don't quite understand the $1k axle? Does it include labor? Isn't this a boxster? Here at PP a boxster S axle is $130. All shops like to make money and they will talk you into stuff you don't need to maximize your visit's bill. Service reps are trained to sell and people do feel good giving their sports car a spa day.
You'll live. Still beats buying a new car or crawling under it all weekend if you aren't into it!
Cheers,
G