The way I see it the time line for the start of the Empire is sort of right in the PT.
It's long ago enough for Luke to have no memory of a world without it but near enough for his Uncle's generation to take sides.
It's established enough to keep most of the Galaxy under it's thrall but still has a large rebellion causing problems.
What the PT got wrong was the length of time it took for the Republic to fold.
Ben talks of Clone Wars (plural) of Vader helping the Empire hunt down and destroy the Jedi, not turning up at their HQ and killing the surprising few that were in residence leaving the Clone Troopers (who are clearly much better shots than their Storm Trooper replacements) to do most of the killing. All this before the formation of the Empire.
To be more accurate the first Clone War should have been shown in TPM, leading to a second when Anakin was old enough to participate both as a Jedi and as a Cyborg assassin.
I proposed a fan edit reason for Luke's comments about Dagobah...he was born there :
