Just to follow up, now that I had the flights:

The outbound DUS-SKG (correction: was SKG-ATH) flights: When I booked 3 weeks prior and 8 seats in C class were available, they denied my waltlist. Fair enough, maybe they thought they would sell them. The day before travel in the morning I called again to have waitlisted again, seeing again 8 seats in C available. They asked me to call back in the evening. I called and waitlist was still unprocessed in the booking.They told Revenue Management (?) which handles the Waitlist are available til 11pm so I shall call back tomorrow. Next morning I see waitlist was taken out of the booking, so they denied the upgrade again.
A couple of hours later in the flight C Class was completely empty. I wonder why they didn't allowed the upgrade, and was thinking that they were also completely empty on the way out and didn't wanted to load catering and have all these extra things to take care of for only one upgraded pax.
I was not impressed by their economy class. While their crew can't be faulted, the hot food was of ridiculous size, smaller than an iphone... I thought this makes a very penny pinching experience. I know it's not what they mean to do, but maybe better rather just give a sandwich...

SKG-FRA: This was upgraded right away after booking. Had a nice flight. Plenty of food, but unfortunately they handed out the menu for the other leg (or heated the food for the other leg, anyway they mixed it up). It was still fine but the original menu they distributed was much more appealing to me

Still great you get so many upgrade vouchers just for retainning/reaching tiers. Very generous