So if the flight is at 8AM is it to somewhere in the EU or directly back to North America or something else? If it’s EU you want to be to the airport at 6. Anywhere else and maybe aim for 5.
So you’d need to leave at 4-5AM at the earliest depending. It’s a 45 minute drive, low traffic that early. I’m conservative on timing, many people will tell you to cut it closer.
I had an early flight out of Athens last year, not on a tour but staying in an Airbnb. I used Welcome Pickups, arranged online, and the driver was outside the apartment waiting at 4AM as scheduled. I’ve also used uber, but they might not be in Athens now (it seems to change).
On a tour the guide or hotel will arrange a taxi for you. It happens all the time, taxis are operating and used to early departures because the airport has early flights.