Yes, as you are saying, ONCE you overcome the pre-load. But that's not negligible, and the pre-load is adding up as you are compressing he spring further..
If the same amount of additional applied force caused the same amount of compression each time, the chassis of the bus would sit on the wheels after 50 people boarded.
Anyways, we might not agree on the springs, but shimming the relief valve is definitely a thing, many engine builders do that to increase the maximum oil pressure. I'm not sure if it makes sense in a Boxster though. I added an extra shim to the valve in my Yamaha and measured an increase of 10 psi in the max pressure.
They even sell relief valves in adjustable versions, where the pre-load is adjusted by a set screw:
https://www.7ent.com/products/adjustable-oil-pressure-relief-valve-kad1011005.html
I found a video of an adjustable relief valve in operation, it's not a car oil pump, but I believe it's the same principle, around 2 mins:
https://www.youtube.com/watch?v=4eXDK7OGcRs