My logic is that we should wait for the sensors to register the present indications so we have a proper idea of what state the engine is in particularly the EGT if we have to consider a manual start.
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Amber X's mean the FADEC is not yet energized. Once the X's disappear it means the FADEC is ready and can provide monitoring (even passive monitoring in a manual start).
Without FADEC there is no means to start the engine, as the valves are also FADEC controlled.
(A320 FADEC schematic.)
FADEC is needed even for manual start.
