End of may should be good. Usually very hot in the air though, you'd want to be in an AC ship. You will still have hammerheads there, but the temperature is up a bit in the water, so you will be more dependent on currents to get them up in ther shallows. Sea is calm, normally, at this time. There can be south winds later during summer and sometimes haboobs, which is sandstorms and rain in a horrible mix coming from the maninland. Normally this occurs during late june/july and to the south, but I once experienced it on Sanganeb - very surreal and viz went down dramatically.
The south is a large area consisting of the North and South Suakin Archipelago. Lots of reefs and small islands jutting up from 200-400 meters. There has regularly (but not that often) been trips arranged to the North Suakins. There are some very nice spots there (Nakkalat El Queseer, Protector Reef, Jibna Shoal, Hindi Gider), but perhaps no Sha'ab Rumi.
The south Suakin Islands have been very little dived, and I have no personal experience from this area. Only a few trips have been arranged here since the 1980's. It is a hard-to-visit-area with no real safe over night anchoring possibilities. Extremely weather dependent and it puts a lot of extra stress on captain and crew compared to the northern itineraries. It has also been harder to obtain permissions to visit this area. But there are a few spots that certainly have a reputation as some of the best diving spots in the Red Sea.
good luck
