What about the small matter of having to purchase a Google phone in the first place?

Most anti-google move: buy a second hand pixel, they receive no revenue on the device which is (assumed) already highly subsidized by google so that they can profit off users' data, then you use their subsidized hardware without running their spyware OS. Google only loses money in this scenario, it is a great protest.

I see it as a necessity, because the Google phone is the only one worth it if you care about security.

The problem is not GrapheneOS, but rather that phone manufacturers other than Google don't care. Now if there were millions of GrapheneOS users, it would start becoming interesting for other phone manufacturers to care.

My point being that I buy Pixel in order to give more weight to GrapheneOS, in the hope that other manufacturers will eventually realise that.

Google makes high quality hardware and untrustworthy software. Graphene's approach is to take the hardware and leave the software.