GrapheneOS doesn't only support Pixel phones because the bootloader can be relocked using a custom ROM (my phone does that) but rather because it is the best phone for security. That much is true. But more importantly, it's the standard Android implementation and the GrapheneOS team only has so much bandwidth to add support for various phones.
However, there's no way in hell I'm giving Google a single cent. Also, I wanted something that could be repaired, and Pixel phones ain't it.
So I went for a Fairphone 4 with CalyxOS. I'm glad I did: the phone is just what I need in terms of repairability (not in terms of price, but that's kind of expected when you buy from a small outfit in Europe) and CalyxOS is a wonderful deGoogled OS for a variety of reasons. Installing it on the Fairphone went as smooth as butter. And yes, the bootloader is relocked :)
One more thing re buying a Pixel phone second hand: it IS equivalent to giving money to Google. You don't give more money to Google, but you partially reimburse the original owner. In other words, you buy back part of their original purchase. In other words, you replace part of their money in Google's coffers by yours.
And if you use a Pixel Phone, you promote the use of Google hardware. I don't want to promote any Google product.
Not to mention of course, the irony of giving Google money for the privilege of escaping the Google surveillance. That's too rich for me. I'd rather have a little less security with a non-Google phone.