Hi Cypress - think you will enjoy your May visit. I well remember my first May visit, especially the transfer from the airport to Sidari in a double decker bus. It was late at night and as we went up the mountain, I suddenly became aware of little twinkling lights in the undergrowth at the side of the road. Of course it was the fireflies and they continued to entertain us at our self catering apartment in Sidari.
It seems to me that May is only slightly different to later in the season as the weather can be a bit more variable. One other point of note is that the olive trees will be in flower and giving off vast amounts of pollen, so if you get hay fever or similar take the usual precautions. On the upside the spring flowers, as you say, will be at their best, especially the yellow broom.
Best wishes
Pat