I am trying to build an autonomous laser engraver, which does not receive data from the PC but instead from another Arduino or an SD card.
Don't you mean:-
I am trying to build an autonomous laser engraver, which does not receive data from the PC but instead from another Arduino or and an SD card.
If not where does the second Arduino get the Gcode from?
Therefore, I do not want any external software like Universal GCode Sender involved
You are going to have to write the equivalent of this on your second Arduino.