The Earth is about 4.5 billion years old and the Sun will expand large enough to make the planet uninhabitable in about 500 million years.
In other words, the Earth is already 90% of the way through its life cycle.
Don't worry though. We'll almost certainly make it uninhabitable for humans long, long before that time
I too flashed it with GrapheneOS and it is pretty great (though some features work differently than the default OS). The long support life is one of the biggest selling points. GrapheneOS has hinted that they will likely support the 7 series even longer than the 5 year window if possible.
In my opinion, flagship phones are quite overpowered for reasonable use cases, so you're better off with a cheaper option. IMO such an expensive phone only makes sense if you need the extra power for gaming or if you really need the higher quality camera.
Maybe you can buy a used 7 series? I'm not sure how the market works in the EU, but in the States people often trade in their old phones when the next model comes out, which is great for the second-hand market. I bought mine on Amazon from a third-party seller.
If after all that you're sure you still want the 8 then you may as well get it now.
Note since you mentioned GrapheneOS: never buy a phone that was previously from Verizon. They're a scummy carrier in the US that locks the bootloader so you can never install another OS. Pixel devices have a 5 digit code that specifies the model variant. Always check that a used phone is not a Verizon variant and therefore ruined by this awful practice
Holy moly I would never spend that much on new phone. If you can, I'd say wait for the Pixel 8a to come out. If you can't wait that long, the 7 series will surely be discounted by now.
I personally got a Pixel 7a for $375 USD a few months ago and honestly I feel I should have went for the 6a which went on sale for $200 right after.
If you're serious that you definitely need the 8, then maybe you're not very sensitive to price; in which case you could just get it today
It's gonna be hard to arrange those services from prison, when the guy's broke
I wonder what the chances are that private Florida island will be washed away by a hurricane within the next nine years