To get a sense of the project and its goals I recommend reading this post[1].
Buying a used Pixel is economical, environmental, and likely doesn't support Google. Pixels are the only secure and open android devices that could work for the project and meet the extensive requirements[2]. This is because GrapheneOS takes real steps to protect user privacy and security, not features that degrade security and don't increase privacy. You are going to be doing much more against Google by using GrapheneOS because it comes with 0 google services by default and takes advanced steps to protect you from all apps and services you install.
If you are still not willing or able to purchase a Pixel, GrapheneOS has a partnership with Motorola to help them create compatible devices which will be available soon[3].
[1] Privacy and security on computing devices need to become far stronger to protect people from pervasive violations of their rights. https://xcancel.com/GrapheneOS/status/2044440381803069778#m
[2] https://grapheneos.org/faq#future-devices
[3] https://xcancel.com/GrapheneOS/status/2028448871374803007#m
The Pixel was never sold in my country so it's much more difficult that you think. Even if it was I wouldn't buy it because I don't buy Google products.
> If you are still not willing to purchase a Pixel for whatever reason, GrapheneOS has a partnership with Motorola to help them create compatible devices which will be available soon[2].
Ok? Wake me up when that happens.
It makes sense that an open source project would focus on one series of phones since their time and resources are limited.
That said, Google's hardware is behind their competitors and they've had a lot of problems in the past few years. The Pixel 8 Pro has hardware WiFi problems, the 9 and 10 are both minor updates with prices that are far too high, the 10 is eSIM only, etc.