Kolbeb Hey there! The 6a has a specific peculiarity and requires a couple of extra steps before you can use OEM unlocking to install GrapheneOS (or any other OS). It is detailed on the installation guide:
For the Pixel 6a, OEM unlocking won't work with the version of the stock OS from the factory. You need to update it to the June 2022 release or later via an over-the-air update. After, you've updated it you'll also need to factory reset the device to fix OEM unlocking.
Kolbeb If I can do so, can I uninstall and restore the OEM settings / software should I decide to sell the phone later? And can I wipe my personal files via a reset without reverting to the original set up?
You can disable OEM unlocking at any point (and in fact, you should disable it after installing GrapheneOS. It's mentioned in the installation guide).
You can factory reset to get rid of your personal files, and you'd be left with a blank GrapheneOS install. You can also re-install the Stock OS before selling it.