I think what happened is that originally, Luke wasn’t going to face the Emperor until Episode 9. Or at least, that format was floated around as a possibility in the very-early 1980s during the very-early stages of conceptualizing Episode 6. Apparently, at a very early stage, the idea was to make Episode 6 an entire movie about the rescue of Han Solo, with Boba Fett as an antagonist. With this format, Episodes 7, 8 and 9 probably never materialized into anything resembling a concrete plot, except that in Episode 9 Luke would finally confront the Emperor at the Imperial Capital. Obviously, none of this happened, and the rescue of Han plus Luke vs. the Emperor were compressed into a single climactic movie, and (later in the production) the Imperial Capital was changed to Death Star 2.
However, I never really thought that ROTJ was too rushed, because I took it for granted that there would have to be many additional battles after Episode 6, before the Empire was completely defeated. The death of the Emperor brought an end to Luke’s arc and Vader’s arc, but the Empire itself would certainly continue to exist in some form for quite some time.
In retrospect, the biggest issue with ROTJ is that the first part of the movie is entirely disconnected from the rest of the movie, and the premise for the latter is mostly disconnected from all events that happened earlier. Nothing that happens in Jabba’s palace has any kind of causal relationship with the events after Han’s rescue. And the entire setup for infiltrating Endor (stealing shuttle Tyderium and all that) is just arbitrarily presented as a given that happened off-screen, with no connection to anything that happened earlier. Consider how in Episode 4, the assault on the Death Star 1 happens as a direct result of the actions of the main characters, whereas in Episode 6, the assault on Death Star 2 is simply presented arbitrarily as a starting premise, not as a logical consequence of prior events.
On the other hand, it’s nice that our 3 main characters aren’t responsible for literally every important thing the Rebel Alliance does. It’s a big galaxy, and there are other heroes going on other off-screen adventures, like those Bothan spies. But from a narrative/writing perspective, the circumstances enabling the assault on Death Star 2 are totally arbitrary. There’s also no well-defined ticking clock. Why should the assault take place now? (Well because it happens that now we have access to a stolen shuttle that was stolen off-screen, and an off-screen spy network that you never heard about before discovered that the Emperor is personally overseeing the Death Star’s construction. That’s why this is happening now, conveniently right after we wrapped up that side quest to save Han.) It comes off as arbitrary, because the premise doesn’t follow causally from anything that happened in the first half of the movie, nor anything that happened in the previous movie.