You should be fine with 2hrs although it would depend on your personal circumstances. I'm guessing you will check in online but would you have a bag to drop? Even so, it will be OK. I assume you're flying with Aegean but what is your status? (oops just re-read your post) It can be VERY busy at 06:30 but bag drop and security are very fast for *Gold, in my experiebce. I usually travel outside peak season but I'm always in the lounge, right by the gates, within 15 minutes of getting off the metro.

The metro is very reliable and (as far as I've seen) runs exactly to the posted timetable. I've taken the first train at 05-something several times. I don't think it takes 50 minutes from Monastiraki. More like 40 I'd say.

Don't be afraid of getting the bus. Those are also reliable and go regularly all night from the main stop at Syntagma (less that 10 minutes walk from Monastraki). They're not very comfortable but always do the job. They can be slow in heavy traffic but would be just fine if you want to go at 5am or something.

DO check local news for the possibility of transport strikes. They happen quite often in Greece but always with a few days notice.

EDIT:

Here's the official metro timetable. The first train from Monistraki to the airport is 05:51 arriving 06:35. You'll be fine! Even if you ended up on the next one (+35 mins) you would still make your flight without rushing.

https://www.stasy.gr/wp-content/uplo...om-24-6-24.pdf