I just noticed that for S13 a new flight has been added: KL1577 ATH-AMS. Schedule:

7:50 AMS 12:05 ATH
13:05 ATH 15:40 AMS

So far it seems only bookable for saturdays and sundays, and no low(er) fare buckets available. (Starting at T)

For me personally the departure time out of AMS is perfect, albeit I prefer to fly out on Friday. And availability in T only is sad (=expensive eco fare), but that will hopefully change in the future.

[edit]
Thanks to airlineroute to provide some more information: From 1JUL there will be 2 daily flights, 9 weekly from 7SEPT. Yay!! For me personally this is a huge improvement!