Vader’s journey to becoming Anakin again didn’t start in Return of the Jedi nor in the time between then and this movie, it started when Luke dropped into the abyss.
When Vader is going on to Luke about how they can team up, kill the Emperor, and rule the galaxy together, he’s pretty much pouring his heart out to him. He’s talking to him the same way he talked to Padme. But when Luke, by his actions, basically says “I’d rather die then join you”, that actually hurts him, deeply, as any rejection would.
Between that moment and the end of Revenge of the Sith, Vader was completely focused on the dark side and being a Sith, but this was the moment that actually brought some of Anakin out of him again. The telepathic conversation between the two pretty much cements this in my view: when he says “Son, come with me”, Vader isn’t talking to him like an evil Sith lord, he’s talking to him like a father.
It’s why he went from being the most powerful Sith of all time, with incredible feats such as ripping open the crust of Mustafar and the netherworld of the Force itself against it’s will (as seen in the comics taking place between ROTS and ANH) to being bested by Sidious (as seen in the new comics; Sidious literally outright states that it’s because he’s lost a lot of his power) and eventually Luke. It created a rift in his connection with the dark side, and he was hugely conflicted.
The arc was completed in Return of the Jedi, but it started here.