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.
Another Roman history buff? Excellent! My personal view is that Roman civilization disappeared starting with the The Last Persian War of 610-628 and then the Great Islamic Invasions 6 years later. The state that emerged at the end of this period a century later was not recognizable as Roman civilization any longer.
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.
My own attempt to create a backstory for the OT that I am working on in the script writing/re-writing section takes this approach. It starts with government corruption and economic difficulties (a widening gap between rich and poor), then comes The Clone Wars, then comes The Emperor. Even in the official canon we get a glimpse of this idea; in TPM Shmi effectively tells Padme that Republic authority has ceased to exist on Tatooine.
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.
It would seem likely that the Jedi would not accept the end of democratic rule sitting down. So the destruction of the order would have to take place before, or almost concurrent with, Palpatine’s assumption of the title of Emperor. However; in the novelization to Star Wars Obi-wan talks about “the later corrupt Emperors”. This opens the possibility to the idea that there was an initial Emperor who was perhaps a benevolent despot - seizing power in order to save the Republic. Perhaps the Jedi served, albeit grudgingly, under the initial Emperor.
Problem is, all of the above would seem to contradict the novels opening crawl where it states that the Jedi had been exterminated at the behest of the corrupt government (the Moffs and Grand Moffs I would guess), that Palpatine declared himself Emperor (implying there were no prior Emperors). Here is the relevant text:
Aided and abetted by restless, power-hungry individuals within the government, and the massive organs of commerce, the ambitious Senator Palpatine caused himself to be elected President of the Republic. He promised to reunite the disaffected among the people and to restore the remembered glory of the Republic.
Once secure in office he declared himself Emperor, shutting himself away from the populace. Soon he was controlled by the very assistants and boot-lickers he had appointed to high office, and the cries of the people for justice did not reach his ears.
Having exterminated through treachery and deception the Jedi Knights, guardians of justice in the galaxy, the Imperial governors and bureaucrats prepared to institute a reign of terror among the disheartened worlds of the galaxy. Many used the Imperial forces and the name of the increasingly isolated Emperor to further their own personal ambitions.
This also implies that Palpatine might not be completely evil. The line that “the cries of the people for justice did not reach his ears” seems to imply that he was being kept in the dark (no pun intended) on what was going on in the Republic. The Moffs and Grand Moffs were the true rulers of The Galaxy Far Far Away.
You have to remember when reading that last part that Palpatine was constantly being duplicitous. His character of Senator/Chancellor/Emperor Palpatine was seen to be easily manipulatable. But as Darth Sidious, he was always controlling things Even the Emperor we get in the OT is an evil and powerful being who could not be controlled by others. He used tricks and the force to get the politicians and bureaucrats to convince him as Emperor to act. So on the surface the Emperor is a puppet, but in reality he is controlling the politicians and bureaucrats who think they are controlling him. An excellent way to maintain power.