Unfortunately ATH isn't the best laid-out airport for Schengen to non-Schengen transfers, and personally I wouldn't want to chance a 1h15 connection on separate tickets if there were other options. Last month I had just under that time on a single ticket transfer, SKG-ATH-LHR, we landed on time (actually slightly early) and I only had about 15 minutes in the lounge before going to the gate at about T-30. That was sitting in row 12 from SKG on a fairly full flight, but the plane emptied pretty efficiently so there wasn't a lot of delay getting out.

To get from Schengen to non-Schengen you have to exit to landside and go back in the departures stream, so another security check and then through passport control (you also get to experience the frustration when you arrive at non-Schengen security of being able to see the area you've just exited). I walk quickly, I know ATH airport reasonably well, so whilst I wasn't concerned about the transfer I equally never felt I had a massive amount of time to complete it.

You will be travelling at a much busier time, queues for security and passport control are likely to be longer so I would regard your itinerary as risky. Even if A3, as is likely, through-checks your luggage you still don't have protection albeit you'd gain a few minutes if they loaded your cases simply because no-one likes doing the paperwork and lifting to remove them from the hold again if they can avoid it.

On separate tickets, I'd want at least a couple of hours between flights, preferably more. If the most important part of the holiday is ATH-IST, I'd strongly recommend getting on an earlier flight if you can.