OK Hervé, I managed to build a Virtual machine on Ubuntu ( macOS Catalina ), downloaded High Sierra, created the USB installer, and installed the latest Clover version ( 5127 ) on it with the customised settings ( Installed Clover in the ESP, Boot Sectors -> Install boot0af in MBR, Clover for BIOS (legacy) booting -> Clover EFI 64-bits SATA, BIOS Drivers, 64 bit -> FSInject-64 / ApfsDriverLoader-64, BIOS Drivers, 64 bit -> GrubEXFAT-64 / GrubNTFS-64 ). One thing to point out is that I don't have the chance to mount the EFI partition with Clover Configurator, I can see EFI straight on the USB ( don't know why, and I hope it doesn't affect the process... ).
At this point I have to set up the config.plist and I have no clue... what are the basics I should set up, in order for this to boot ? Is there any guide I can follow to set it up based on a given system ?
Thanks