IMHO, Lucas always intended sequels. Initially, Star Wars was the first episode in the projected 12-episode saga called The Adventures of Luke Starkiller. And as evidence that it wasn’t just because Star Wars was successful that sequels were made, Lucas hired Alan Dean Foster early on to write a potential low budget sequel story to be used should Star Wars underperform. (That story was released as the novel Splinter of the Mind’s Eye. Foster also ghostwrote the Star Wars novelisation.)
But when Star Wars was successful, serious thought had to be put into the entire structure of the saga. That’s when Lucas decided that Star Wars was Episode IV (out of nine episodes).
Nine episodes was also maybe too optimistic a commitment. After Lucas’ marriage failed, I think that he was burnt-out and resented the fact that this monster he’d created had destroyed his life. It wasn’t until he worked on Jurassic Park ten years later that he got the taste back.
I think the six films we got are broadly faithful to ideas he generated in the '70s. Threepio was built by Anakin in his original notes (which also include ideas about midichlorians), and Obi-Wan and Anakin always dueled amidst lava. As early as 1978, Lucas was discussing a garden planet (Naboo), a rock planet (Geonosis), a water planet (Kamino), etc. The Kaminoans first appeared on Bespin in early drafts of The Empire Strikes Back.
And if you read the Star Wars novelisation, the prologue gets the arc of Palpatine’s rise to power largely correct. The “ambitious Senator Palpatine” was “aided and abetted” by “the massive organs of commerce” and elected to power. “He promised to reunite the disaffected among people and to restore the remembered glory of the Republic.” However, “he declared himself Emperor” and then “exterminated through treachery and deception the Jedi Knights”. It all fits with the prequels.
So Lucas certainly knew the rough outline of the saga and many details he wanted to include, but the actual plot probably only crystalised as each film was made.
Honestly it seems to me Lucas repeated (or foreshadowed) the Indiana Jones syndrome when he told Speilberg that if he took to directing Raiders of the lost Ark, he would be entering at least a three movies franchise. Of course as soon as the success of Raiders cooled down, and Spielberg turned to Lucas for the follow up, Lucas had actually little to nothing to follow and had to hire some writers to do the shit Lucas was supposed to do and was bragging about (I cant help but notice that while Lucas was credited for being the mastermind behind the plots of SW or the Indy series, in truth, he just put down some really vague ideas and never did the real work himself). Lucas is probably a very good producer, but IMO he will always be a very lackluster director and writer.
Yeah, he supervised sound effects. I also have some recollection of him shooting some second unit material uncredited, but I can’t find a source for that.
Find the story conference transcript for Raiders. Lucas has so many ideas for the movie that many of them ended up resurfacing in Temple of Doom and Last Crusade. Read the transcript and you’ll see what a great storyteller he is.
He’s certainly no screenwriter. He admits to hating that part. But as an ideas man he’s fantastic, and, really, ideas are a good chunk of the writing process.
Even though anybody can come up with “an idea for a movie,” not many can come up with fifteen ideas that would all fit seamlessly into one story, and then assemble them in the order that would structure it in the most compelling way, but Lucas can do that effortlessly. Then, if he hands it over to a writer who can put them together and write great dialogue, it’s a winner. The problem comes when he vetoes or otherwise interferes after the fact, which is one of the reasons Crystal Skull was such a mess.
Coming up with ideas, as any writer would tell you, is the fun part of writing. Structuring the ideas, making them click together, fleshing out characters is when writing actually starts. It’s like a director that would never wake up at 5 AM to set up his shots and would just come when everythig is ready to say “action”. Sorry, that’s not the job.
So far, having good ideas for a film but not the commitment nor the energy to film yourself was the definition of a producer in classical Hollywood. That’s why I say Lucas is probably a gifted producer, he hasnt got it in terms of directing nor of writing (Christ , just look at the diff between Empire strikes back and Return of the Jedi on one side and a New Hope on the other side, directed by Lucas).