If you don't manage to factory reset, there's a good chance the repair store will factory reset the device anyway. Not sure how exactly they do that, but that's what happened when my friend got their Pixel 6a screen replaced. And that was on stock Pixel OS which has factory reset protection, so I imagine they'd be able to do it on GrapheneOS too.
If you still want to do it yourself for piece of mind, follow these instructions I wrote using my own GrapheneOS device to factory reset the device:
- Start with phone powered off
- Hold power button and volume down to boot into bootloader interface
- Press volume down button twice
- Press power button once to boot into recovery mode
- Wait 20 seconds for phone to boot, that gives enough time for your phone to enter recovery mode
- Hold power button and press volume up button once to move to recovery mode GUI
- Press volume down button four times
- Press power button once to select 'wipe data/factory reset'
- Press volume down button once
- Press power button once to confirm factory reset
- Wait a few seconds for phone to factory reset, not sure if it automatically reboots or goes back to recovery mode after. If it goes back back to recovery mode, just press power button once to reboot
Adapted from a previous post I made: https://discuss.grapheneos.org/d/3952-cracked-screen-best-way-to-wipe/7