>>1953709
The original Star Wars novel, technically released before the first movie, gave a basic outline of the prequel trilogy, even specifically naming things like the Sith, Chancellor Palpatine and Coruscant, which aren't mentioned in the movies until Phantom Menace. But Lucas did have the basic outline the whole time. I forget if there's evidence if he actually thought of Darth Vader as Luke's father until after making the first movie, but nobody complains about that one retcon. Also, his name literally means Dark Father, and I think that's a pretty glaring piece of evidence.
There are even notes Lucas made before Empire Strikes back that outline the sequel trilogy, as well as three more standalone spinoff movies. These are much less fleshed out, as Lucas didn't seem to care about them as much (and the spinoffs even less than the sequels), but they're there. The original six movies were based on his initial draft of Star Wars, which was the basic story of all six, with the rise and fall of the empire. Of course it changed somewhat significantly once he realized it wasn't practical to make it as one movie, with things like Luke and Anakin becoming separate characters, but the basic outline was there.
The sequel trilogy and spinoffs seemed like things he started thinking about once he saw the first movie was a massive cash cow, and he started trying to think of ways to milk the hell out of it. Like he was never planning to direct the spinoffs himself, but once he tried not directing Empire and Jedi and realized he cared too much to hand them off, he seemed to lose interest in spinoffs, as he realized they wouldn't be as low-effort for him as he thought. One of those standalone spinoff movies, about Chewbacca and his family, seemed to form a very loose basis for the Holiday Special, and that probably also soured him on further spinoff movies, at least live action ones.
And as Disney likes to point out sometimes, the actual sequel trilogy is extremely loosely based on Lucas's actual idea. There were going to be two protagonists, Han and Leia's son and daughter, and they were going to be trained by Luke as the next generation of Jedi. Those characters were distorted into Rey and Kylo Ren (Lucas's versions had different names, not to mention significantly different personalities and stories). Lucas did fiddle with the sequel notes more once the Disney deal approached, though. Like in the '70s, his basic idea was that The Emperor would survive and then be truly defeated in the sequels. By Return of the Jedi he decided against doing the sequels at all, and just killed The Emperor (though maybe at one point he was planning to make it look like he died here and just bring him back later). By the time he sold to Disney, his idea was to have Darth Maul come back and be the main antagonist. This is why he had Darth Maul come back in The Clone Wars. While he was pretty hands-off with that, he occasionally provided ideas. Another one was an arc that actually dealt with The Whills, who are originally referenced in the first novel, which I remind you came before the movie (but was based on his screenplay).
I recommend a Youtube channel called Rick Worley. He has an extremely in-depth series called How To Watch Star Wars. One part in particular is about the development of the series, which is relevant to this discussion. Other parts are about things like why the Special Editions and Prequels are legitimate. All the parts go deep on things like themes and symbolism and filmmaking and Lucas's plans, style, and intention.
>tl;dr: Star Wars is deep for real and almost all of the retcons and depth were planned very early, if not before the original movie, then before the first sequel, which everyone loves. Even the sequels were planned before the original trilogy was finished, even if Disney bastardized them nearly beyond recognition.