When it comes to the fall of Rome, historians will typically say something about how Rome never really fell, it slowly faded away over centuries, but if you really need a specific date, here are some good candidates (235 AD, 476 AD, 1453, etc). One of these is usually used as THE date Rome fell (476), but almost always with the clarification that that’s not actually how it worked out.

Maybe it could be something like that with the Galactic Republic/Empire. Here’s the date where the Jedi were officially declared enemies of the state. Here’s the date where the position of Emperor was created. Here’s when the Republic began imperializing the Outer Rim. Here’s the first major policy shift away from democracy toward autocracy. Et cetera.

Maybe the date where the Jedi were declared enemies of the state is the 476 AD of the Republic/Empire. Generally accepted to be the turning point, where yesterday you had the Republic, today you have the Empire, but it’s just a convention created millennia later. Not all that much actually changed, and other historians in other contexts would use different dates for different reasons.