THE MOST DIRECT ROUTE TO AMORGOS IS A FLIGHT INTO SANTORINI, THEN A HIGH SPEED FERRY TO AMORGOS. Delta will fly into Athens and you can get a 50 min hopper to Santorini if you'd like to explore that area as well. The ferry schedules aren't released yet, so if you do book a flight, make sure you allow for flexibility to arrive in time for the retreat start on the 22 and to stay until after closing on the 27th. I would suggest to arrive in Santorini by April 21st so you can possibly get the early ferry on the 22nd to Amorgos. (There is a high speed ferry from Athens to Amorgos but it is 5.5 hours or up to 9 hours on a slower ferry or in less ideal conditions)
Keep in mind that an international layover could be a great way to see another area during travel! London and Paris are common options, just check the times to ensure you have time to leave the airport with time to return before departure.