Well, the more advices you will have, the best for you... So I wil be a little affirmative...
In my opinion, the best period will be between May 15 th and end of June.
Sailing west could be difficult at any moment in these area, but it will be easier to find "windows"...
The route will be W through the Cyclades
. Wonderful cruising at these period : not too much people and flowers everywhere int the islands (Delos is one of the most interesting archeological site in the world)
Don't sail more south than Milos to join after the Saronic Gulf and Korinthos Canal. Then you will have a lot of shelters in the Patras Gulf (Delphes is the other "most interesting archeological site in the world" and accessible from Itea or Galaxidi).
You will arrive to Kefalonia and then north to Croatia
. But you know, north of Kefalonia, Ithaque is the island of Ulysses !!!
A wonderful sailing trip indeed. Enjoy it !