GrandJambon I recently installed graphene OS and after only about 3 days of using it, upon a power on it did the classic 'optimising apps' message and then reboot itself again, only this time it got stuck in a boot loop [...]
I think app compilation is expected to happen only during an OS upgrade, but an upgrade can't happen while the device is powered off. Can you provide information about what happened before the device was powered on? Had the battery run flat?
GrandJambon Loading into recovery mode from the fast boot menu doesn't seem to do anything, it just restarts the same loop I've already mentioned.
If the device is boots less and less far before restarting, that suggests a possible hardware failure.
The original post doesn't say which device model this is, whether it is new or used, how long you've bad it, or how long it was observed to run OK under some other OS. In general, more details might be useful.