I’d like to share how the issue was finally solved: after replacing the extruder board, changing the extruder three times, and testing two filament motors purchased from Aliexpress with no success Creality sent me a filament motor. Only after installing the motor provided directly by Creality did the printer start working perfectly.
Summary: even though the two Aliexpress motors looked original, neither of them worked. The problem was only solved with the motor supplied by Creality itself.