The Armana letters show that Egypt was not there in Canaan fighting to enemies of the Canaanite states Egypt had conquered. I guess the forts and mansions etc were still there but the presence of the helping army of Egypt was not there while Joshua was conquering around 1400 BC.
This is just not true .. from 1400 BC to 1200BC Hittites of Anatolia and Egypt .. fought to a stale mate and everything in between was Vassal states.. by "everything" however .. we don't mean "Everything" .. it is not like today where you have nations .. you have individual city states ..each controlling the surrounding lands .. in between these city states you have murading bandits .. .various tribespeople .. Goat herders.
The area around Jerusalem was a large trading route .. the city states did well .. paying tribute to one or the other .. and going about business .. the occasional skirmish here and there. One of the biggest things that was traded was Humans .. Slaves. it is a slave economy and one of the good places to get fresh slave meat .. are these tribal people living in the margins .. often there is talk of raids of the country side from these tribal people from the hills .. often gaining power over city states .. and even nations .. such as Babylon which was run by the Kassites for 300 years. One of these Tribal bandit groups were the Hapiru .. in all likeli-hood these or similar groups coalesced and became powerful during the Bronze age collapse .. happening round 1200 BC
So your story is like this Lads .. sure there were lots of Semetic Slaves in Egypt .. perhaps some left over from the Hyksos rule round 1500BC .. short period where semetic Kings ruled egypt .. alowing migration of semetic peoples .. who were then either booted out or left willingly as when the Egyptians took back the Throne the Semetic folks were not treated well. and/or -- there were alot of slaves in Egypt from these tribal people in the margins.
When they left egypt ? .. well .. they never were all in egypt to begin with but certainly any migrations of tribal people out of Egypt would have coalesced with the existing. Tribal people .. warring and battling amongst themselves as much as with the bigger powers.
Then come the Bronze Age Collapse -- within a very short time the Hittite Empire disappears from the face of the map .. Assyria and Babylonia recede - as does Egypt .. under seige from the Sea Peoples .. weakened due to some massive climate event .. which hit the entire known world.
All of a sudden you had a distopian world of anarchy .. no established powers directing the show .. These tribal folks -- perhaps the folks best suited for survival of this apocalypse .. rise to prominance for a short period of time .. David Takes Jerusalem around 1000 BC .. a Tribal War Lord who managed to unite 12 tribes for a short period of time .. the fun lasted for a few hundred .. the Old powers recovering former Glory Assyria comes knocking for trubute round 750 .. 10 of the 12 Tribes are wiped out and dispersed to the far reaches of the Assyrian Empire by 720.
Read your reference again you either cannot read or you are lying. It covers a range of historical conquests. The conquest of Canaan was between ~1500- 1200 BCE. as previously referenced citing evidence which confirms what is on the Stele.