Imagine thinking that Yoda just not ever involving himself into anything is more interesting. That would definitely make him quite hypocritical when he says “Do, or do not, there is no try”. He doesn’t say, “You’re better off just watching everything bad happen from a distance and not do anything about it.” Him knowing Anakin more personally is way more impactful and makes for a better narrative. It makes it so Yoda is more right to be cautious about training Luke. He shouldn’t just sit out as the entire galaxy goes to shit and the Sith take over. Like wow, what a great hero.
It’s not the EU that “rehabilitated” them, he just learned between the movies. It’s established in Revenge of the Sith that he became Qui-Gon’s apprentice, it doesn’t take much to realize that the change in his character between the prequels and the originals was because his perspective changed during his exile. There’s not even any EU stuff that covers what Yoda did between ROTS and ESB. Guess who also had character development between films? Luke, between ESB and ROTJ. He grew from rash and impatient to collected and wise. But I guess that wasn’t actually in the movies.
He didn’t say “Don’t underestimate the Emperor’s ability to manipulate people”, he said “The powers of the Emperor.” It’s a clear foreshadowing to his force lightning anyway, as the moment Luke lets his guard down and underestimates the Emperor, he gets attacked by force lightning. Force lightning is the payoff to all the talk of the Emperor being someone you don’t underestimate. And Yoda knew this, because force lightning was how he lost.
First, Palpatine shows more powers to Yoda then lightsaber dueling. He uses telekinesis and force lightning too. In fact the first thing he does is force lightning. Second, the force in the prequels is the same as in the original movies. They didn’t introduce anything new. Force lightning, telekinesis (Vader throws objects at Luke in ESB, force choking is technically telekinesis, Yoda lifting up an entire X-Wing), force speed and super high jumping (Luke did it in ESB AND ROTJ when he jumped up to avoid Vader, and Vader also did a big jump in ESB), seeing into the future, fast lightsaber dueling (sometimes Vader and Luke’s duels can actually be pretty fast in their most intense parts; the only reason they weren’t faster is because Luke is a novice force user and Vader isn’t trying to kill him, it makes a ton of sense that Jedi masters that trained for decades would be incredibly fast and powerful), it’s all there. The force giving people “superpowers” is not something the prequels introduced. They’ve been there from the beginning. The prequels just showed far more experienced and powerful force users more often using their powers (often because the circumstances were more right in the prequels to show off their full power; Vader was incredibly powerful but had to hold back the vast majority of the time). They’re just the original powers dialed up because they feature more powerful, experienced people who are more skilled with those powers more often in circumstances where they should use their full powers. Stop pretending the original trilogy didn’t have any force superpowers. Don’t pretend Yoda didn’t lift an X-Wing, Luke didn’t jump super quick high up in the air to escape the Carbonite freezing, Vader didn’t choke a guy that was in a different location then him and threw things at Luke, and that parts in his fight with Vader in ROTJ actually go really quick. Third, the prequels are nothing like Dragonball Z. They have just as much in common with them as the originals.
Also the force in the prequels isn’t just superpowers and fights. The Jedi fight as a last resort. Do you not remember everything Qui-Gon said about the will of the Force and how the Force will guide them? Literally the first discussion in the entire prequels is Qui-Gon telling Obi-Wan about needing to be “in the moment.” Qui-Gon was overall very elegant and faithful. I guess you also conveniently forgot about Anakin and Yoda’s talk about how he should “rejoice for those who transform into the force.” All the times they say “May the force be with you”? “Balance to the force”? The force is still very much a spiritual thing. And no, midichlorians do not remove that. They’re the bridge that connects living beings to the force, NOT the force itself.