USB Debugging isn't necessary for installation. Follow the instructions on grapheneos.org/install/web ONLY do not use other guides independently.
Only developer option toggle required to be on is OEM Unlock.
Also keep in mind that the 4XL is End of Life:
The only thing beyond this is your fastboot driver needs updating or you have a problem with sonething else is controlling the USB connection. Reboot the PC you're using only opening the web installer tab and make note of any Error messages and report back.
For the easiest install use another modern android device.
Any updates on GOS-native device project(s)?
What about One+?
This was the position regards One+ back in 2022 and the same applies today:
"OnePlus doesn't provide anywhere near the level of security features and alternate OS support we need, and we're not into making substantial compromises where we can't provide the level of privacy and security users expect from GrapheneOS. We expect to need collaboration with OEMs for that."
As far as that collaboration for native GrapheneOS with an OEM goes, discussions continue with several possible avenues and we will continue to be open to discussions.
