Nazi comparisons within Star Wars have their limits though. Or at least I've always felt it's something people hold onto too much. Visually there are some comparisons that are obvious and some subtle... The term Stormtrooper transcends an SS Stormtrooper from Nazi Germany though, and I've always felt the Empire's Stormtroopers were more akin to the Stormtroopers of World War I and their overall purpose, but with a mix in of some of the German SS Troops of WW2 as well.
In WW2 though, the Germans weren't an empire to start... The Empire in Star Wars, ultimately it is an Empire already. It's just rebranded. The Republic Senate becomes the Imperial Senate, and that isn't even disbanded until ANH and the Death Star's completion.
The Galactic Empire, at its onset, was a welcomed sight to most as well... Nazism actually wasn't a popular political movement at first though, and didn't take hold right away by any means.
Regarding the birth of the Galactic Empire though, Like Padme says, "This is how liberty dies, to thunderous applause". The Third Reich was more born out of the way WWI ended for Germany.
In Star Wars I feel like the Empire's born at the end of ROTS obviously, people seem to want it and it seems to be a popular idea... The war's over, and the only people who'd be against it would be the losers of the war ultimately, and as the Empire settles in and begins expansion I would assume it's where they expand that really get annoyed by it.
There's cut footage from ANH of Luke/Biggs discussing the nationalization of the planet... That before long Owen would be another tenant farmer under the Empire's thumb. Likewise, in TPM Tatooine's established as separate of the Republic and even far worse (they seem like a dangerous place, the Hutts appear to be inherently corrupt and dangerous, and slavery is tolerated here). By ANH we see the Empire has obviously expanded to this planet, and overtaken it to some degree... again, for better or worse. Obviously some would support this, but people like Owen would hate it once he'd basically be farming for the greater good and not for himself.
Add into all that, the galaxy is literally millions of these worlds, if not billions, or more, who knows really, and the Empire seemed to have a VERY firm grip and support just at the end of ROTS... In galactic terms that's not all that long I'd think really for the Empire to have solidified things by ANH to the point that the Rebellion is small (on galactic terms... Small could be literally billions of soldiers and maybe hundreds of planets supporting it, and it's still just dinky compared to the Empire's side of things).
Flash forward the time from Jedi to the new trilogy... The Empire and Alliance could really theoretically just be on more even footing is all.
One has to assume immediately after Endor there's a certain fragmentation. Palpatine probably didn't have a solid plan for a successor I'm guessing. Just prior to and after Hitler's death, there was a lot of jockeying for position as to who was going to be in charge and call the shots... It was also a crumbling situation though, so it's again not a direct analogy.
I'd think though it's reasonable some higher ranking people people within the Empire looked to take control, but there'd also be a lot more to take control of too. You lost one fleet (a major one) and a 2nd Death Star, and your supported leader, by this point. You're hurting, but someone is bound to be competent who could take charge and unite as well. There are probably tons of people to pick from. That was always the Thrawn Trilogy angle... Grand Admiral Thrawn united things, and turned it around.
Evil Empires have disappeared quickly in our past because they've largely been beaten to a pulp by someone else at that point and just disappeared. Think about the British Empire's decline though... Far slower, and maybe more of an analogy perhaps? Fell via rebellion to some degree... not totally by any means, but some.
Just some thoughts though. I think a galactic conflict, Galactic Empire, and its obviously galactic scale means it could have a galactic-sized timeline to it too that really transcends most anything in Earth's history... The Empire could linger on for at least decades in some form or another.
I kind of found the 3 year span of the Clone Wars comical because it's so short. I wish we'd seen it last somehow from TPM to the end of ROTS's timeline. Much longer, much more destructive and impactful around the galaxy.