Originally Posted by Carpacchio

Those are exactly the key questions:
if you have luggage and it's not checked through, you need to get it in ATH and recheck it with Aegean.
However, that means you need to enter Schengen zone and hence need a visa for it and fill in the Greek PLF (I assume you are vaccinated or have a negative test).

If not, you can stay in the non-Schengen zone (as it seems that the flight will leave from the A gates which are non Schengen) and would need to indicate "transit" on the PLF.