Most experienced travelers to Greece ALWAYS spend the night before an
international flight back in Athens
Partially wrong.
Most experienced travelers know the risks of returning to Athens by ferry on the same day as the return flight home. (and possibly by plane from a small domestic airport such as that of Milos with its East-West oriented runway). This risk with ferries in Greece is well known, I have practiced it for more than thirty years.
On the other hand, in the case of a departure from Santorini with a stopover in Athens, the risk is no greater than for any other stopover in Europe.
At Santorini airport, last year 47% had a delay greater than 15 minutes, the average length of delay was 22 minutes.
1% of take-offs were canceled, out of 15,652 take-offs.
This is less than the 1.8% of flights that were canceled or diverted in U.S.
(at Milos airport 9% of flights were canceled. In this case it is a risk to be taken into account)