So this could all be based off false info but as far as I can tell there was a period from 7500 BC to 3000 BC referred to as the Neolithic Subpluvial. All information I see points to this affecting North Africa, including Egypt.
The Great Pyramids seem to have begun construction in the Early Dynastic, which goes from 3100 to 2686 BC. Meaning they were constructed when Egypt was tropical rather than desert, if I understand correctly.
If that's all the case (and here's where I feel like the question gets dumb/crackpotesque), is it possible that the pyramids were created not primarily for awe or symbolism, but rather to seal out humidity and preserve the bodies within? That they prepared the bodies, put them inside, perhaps used salt to absorb moisture, then left sealing the chambers and corridors as they exited to seal out the moisture?
Also given the climate/landscape difference, how likely is it that the pyramids were partially or perhaps mostly underground?
As far as we can say now, the Sahara was a dry as now by 3400 BC at the latest. I
It seems to be no coïncidence that around this time we see neolithic cultures colonizing the Nile valley. In the perod of the "wet Sahara" you describe, the Nile valley was not an agreeable place to live: inundations, swampy, etc....while what is now the sahara was lush grasslands (not forests as you mention)
Concerning the "colonisation" of the Nile valley we are unsure at this time wether this happenend by the people living in the lush Sahara before that, or people bringing new technologies from the fertile cresent.
There are very strong indications that some forms of farming and especially control/draining of floodplains is a technology brought from the fertile cresent. As usual the answer lies probably somwhere in between: populations going more and more towards the Nile valley as the desert dried out meeting with migrants from further east.
The desertification of the Sahara took a very long time and as the Sahara became more dry, we see the rise of more complex neolithic groups in the valley itself, which now had become a better place to live than in the dertificating desert.
It seems no coïncidence that according to the latest evidence, we begin to see a population increase, an increase in making the Valley useable for agriculture and the first proto states from about 3500 BC. (and we are no sure wether this proces started even before that)
By the time the pyramids were built, permanent human settlemnt outside the Nile valley was already long gone. The desert was as dry as it was now and the pyramids were built in this desert.
for a good reference on the forming of the state in the neolithic to early dynastic period i can higly reccomend "Ancient Egypt: anatomy of a civilization" by Barry Kemp, which devotes large parts of the book to the forming of the Egyptian state. (and the rest of the book is also a very good read on the Egyptian civilisation down to the Amarna period.)
To conclude: by the time the pyramids were build, the desert was as dry as it is now.