I wouldn't do it. I hate crowds, and I would rather endure the crowds than the heat. Cairo will be hot, and it's just going to get hotter as you go south. Most of the attractions are outside with very little shade anywhere.
Here are the average temperatures you will be looking at in June:
- Cairo: Mid 70's at night to 95° during the day
- Luxor and Aswan: Low 80's at night to 107° during the day.
I have sailed on dahabiyas twice. Although I am a huge fan of dahabiyas, I wouldn't do it in the summer. You will likely spend most of your time during the day on the top deck, which will not have air conditioning. In addition, a lot of dahabiyas don't run electricity, or only very minimal electricity, during the day because all the electricity comes from generators.
One more thing about dahabiyas. I would recommend a southbound dahabiya from Luxor to Aswan instead of a northbound dahabiya from Aswan to Luxor. The winds favor the southbound dahabiyas. The southbound dahabiyas are much more likely to travel by sail, and the northbound dahabiyas almost always have to be pulled by a tugboat.
One of the good things about dahabiyas is that they usually try to time the stops to miss the crowds from the big cruise boats. We were the in shoulder seasons, March and October, and we pretty much avoided crowds. Karnak Temple in Luxor was crowded, but the dahabiya stops -- Esna, Edfu and Komombo --- were not crowded at all. (The Giza Pyramids will almost certainly be crowded. We inadvertently missed crowded at the Pyramids because we were there in March 2020, just before the great Covid shutdown.)