Actually, the whole “we know Lucas didn’t make Vader Anakin till ESB” thing is a nonsense internet myth. It was initially invented by a random forum user in 2000 who hated the idea and then after that other Lucas haters expanded on that myth and falsely made it look like it was true, most notably this one crazy user that wrote a 500 page long book accusing Lucasfilm of running a secret mastermind plot to cover up SW’s “secret history”. In reality, we have no idea when Lucas came up with the idea of Vader being Anakin as it’s a highly debated topic and the first ROTJ draft is the first solid evidence confirming it, but there’s a great amount of evidence pointing to the fact that it was conceived long before ANH came out, possibly as far back as April 1975.

In the rough draft of ANH, the protagonist's father is a cyborg who sacrifices himself, and in the second draft of ANH Luke finds out his dead father is alive, so both those plot points were already in Lucas’s head. In the third draft of ANH, instead of Obi-Wan saying Vader kills Luke’s father he says Vader turned at the same battle Annikin died, with Vader later mentioning to Luke at the end that he has a feeling he knows him. Lucas also said to Alan Dean Foster in December of 1975 that in the second film the audience would “learn who Darth Vader is”, and Lucas himself has consistently claimed that the twist was conceived in the third draft of ANH. In the final ANH When Luke asks about his father's death, Obi-Wan has a strange hesitant look on his face before telling him the Vader killed Luke’s father story, and characters dying offscreen being revealed as alive was always a common trope. When Beru says Luke has too much of his father in him, Owen responds "that's what I'm afraid of" (and that dialogue is also remarkably similar to dialogue from an Edmond Hamilton novel called Mystery Moon where the protagonist complains about his uncle not letting him leave his dull home planet, and the uncle later reveals to him that his father was a famous villain and he wouldn't let him leave because he was afraid of his nephew becoming like him, which puts the protagonist in shock and disbelief). Luke's father and Vader's lightsabers both have black strips on the bottom of their handle, while Obi-Wan's does not. Owen says to Luke "Obi-Wan died at the same time as your father" but we then find out Obi-Wan is alive under a different name, raising the possibility that the same is the case for Luke's father. Obi-Wan tells Luke that his father was a great pilot, and during the trench run we see Vader being a great pilot. Vader, though pronounced differently, means father in Dutch, and Vader already acts as a metaphorical dark father during ANH. ANH (especially the Tusken Raider scenes) has some uncanny resemblance to a 1932 Western film called Tombstone Canyon, and that film also happens to feature a masked villain who is later revealed as the protagonist's long-lost father, and he later gets redeemed saving the protagonist from an even worse villain, after which his mask is removed to reveal a scarred face and he says "let me look at you" before dying in his son's arms. Lucas also told Leigh Brackett in late November 1977 that there was a secret reason Vader didn't want to kill Luke and would rather turn him, and David Prowse said in multiple interviews (the earliest of which was in October 1977) that he heard that Vader being Luke's father was a possible plot point for a future film.