Thank you all!
Phead I used Arch to install GOS, so in case you're in for distro hopping Arch would be a safe bet.
Yes, ArchLinux would have been worth a try too. I originally used this to install Graphene OS.
Equal2024 Edit: Ah, I see you're referring to whether they've used the Flatpak or Snap versions of the browsers; my bad. This might be the case, as they've used Fedora and Ubuntu. In addition to this, using a containerized version of a browser is not a good idea anyway, because it replaces the much more robust browser sandbox these browsers are using with the Flatpak or Snap sandbox; see some discussion here:
I used the Chrome browser as the browser. I installed Chrome directly and neither as a flat pack nor as a snap. But it still didn't work.
Equal2024 Here's one thing you haven't tried yet:
Caution: If you have Android Debug Bridge (adb) installed on your machine, stop the adb service with the command below before proceeding as the service interferes with the flashing process.
adb was not running at the time I tried this from different Linux systems. I had checked that. But it still didn't work.
The only thing that worked was finally installing Windows 11 and doing the whole thing from Windows. It was a hassle to install the correct drivers, but I managed.
Equal2024 @Thermos4940 Though this is unlikely to be related to the issue you're facing, make sure you've removed the GrapheneOS key before using the flashing tool; text from GrapheneOS web installer:
The GrapheneOS factory images flash a non-stock Android Verified Boot key which needs to be erased to fully revert back to a stock device state. Before flashing the stock factory images and before locking the bootloader, you should erase the custom Android Verified Boot key to untrust it
Having already flashed Stock Android I almost forgot to wipe the Graphene keys, but luckily I remembered that at the last moment so I could do it before locking the bootloader again.