Flights to Greece are expensive, especially the nonstop ones from the US and of course flights in 2023 are very very pricey in terms of points/miles and OOP. It's a good idea to look and get an idea and save/squirrel away more than you need, just in case things cool down but rarely does that happen, frankly. Then, if you want business class you're adding on even more expense, so you're not doing anything wrong, per se, you're just experiencing sticker shock, lol.
I'm always searching 2 years out and keep a notebook with goals and the flexible miles to attain them as well as Plan B and Plan C. For instance, in 2019, I had a goal of traveling to Australia in biz or better with DH and my list included - Plan A- United 400K miles, Plan B AA 220K miles, Plan C Delta 500K miles when an alert came up for 2 seats available one way on Qantas F via AA for 220K AA miles, which I had, so I was able to book those flights (and then booked a cruise around the flight, lol).
ETA- I could have transferred United miles from UR and Delta from MR*
On the bright side, it sounds like you have lots of time to play with dates and your cruise isn't locked in, so you could fly to a cheaper airport like Rome, Barcelona, Lisbon or Paris, spend a night or two there and then fly on to Athens and do the same on the return or do an Athens to Rome cruise and then fly RT FCO and fly more cheaply in and out of Rome (or IST or...).