Thank you MaLd0n for continuing to try to help me.
The good news, with your DSDT and the SSDT-UIAC I made under Clover, I finally managed to boot with OC 0.6.5. I didn't dare before to put the DSDT.aml in Opencore's ACPI folder, because it's not recommended in the Dortania guide I started with - why does it do that anyway?
I started with just CSM, CFG-Lock and Serial Port disabled and here I am, I still have this "disk not ejected properly mac" message with another usb key plugged in. The first one by the way gave up the soul, I don't know why?!