Hat Having an alternative to Pixels will of course be beneficial for everyone. I also think that some users would like to support the Foundation by choosing this device.
In the current landscape not a lot of phones have good hardware security, and also not a lot of phones support verified boot of third-party operating systems, if an OEM is willing to work with the GrapheneOS project to meet the security standards, fast patch turnaround time, etc., that will be of substantial benefit to the GrapheneOS project (and, I suspect, to the OEM).
If some users are willing to donate to the GrapheneOS project, there are multiple ways to do that. And those ways don't have the weird corner cases that running money through an OEM would have. For example, what if a user buys one of these devices and returns it after 90 days? I guess the OEM could delay passing a donation on for some months, but that's not exactly a benefit. All of this assumes that the OEM would somehow know which of the devices are running GrapheneOS, which seems unlikely -- I have not heard of any plan for the OEM to sell GrapheneOS-only devices or even to sell devices with GrapheneOS preinstalled.
The GrapheneOS donation page currently has eleven options, all of which work for people who own Pixel devices, and all of which are likely to keep working for owners of a non-Google device.