besen
Did you hold volume down the entire time? You have to start holding immediately after google logo shows up (the sequence should be alternate os warning -> google logo -> grapheneos logo), and keep holding until the phone's fully booted.
Failing that, you could try adb sideload to reflash the OS. Power down the phone and hold the volume down button to get access to the bootloader/fastboot screen. It should look like this:
https://storage.googleapis.com/support-forums-api/attachment/thread-282377112-7952556128442379489.jpg
Then use the volume buttons to select recovery, and once you're in recovery use power button + volume up to bring up the menu, select adb sideload, and use a computer to sideload an ota zip.