Hold down the power button until the phone turns off. Once it's off, turn it back on while holding volume down and you should get to a screen with a triangle and ! with "Fastboot Mode" underneath it.
If you see "Device state: locked", use this site to restore stock firmware: https://pixelrepair.withgoogle.com/
If you see "Device state: unlocked", try to continue with the GrapheneOS web installer from the "Download release" button.
Personally, I would try using Chrome or Chromium over Brave. I'm not sure how to properly disable shields to get the GOS installer working on Brave as I don't use it.