Recently I had an issue where I could not boot GrapheneOS, it gave me an error that no OS was installed, sometimes it did boot again but the issue persisted randomly at boot, I could also not flash GrapheneOS through the website or manually, it simply did not write the main OS, I could only flash an update manually.
After that I flashed Google's stock OS through Googles website, and GrapheneOS could be installed just fine after that, it seems to have "fixed" something.
Perhaps you could try to do the same thing.
As long as you do not login to Google and keep the device offline it should be private enough.
After that try flashing GrapheneOS again, it worked for me. (Don't forget to turn on OEM unlock in the OS as well as USB debugging in developer setting before flashing)