The ST is about stopping their respective returns.
Except they didn’t stop them. Canonically, Palpatine inhabited his new body on Exegol almost immediately after his death in RotJ. Palpatine was still alive for that entire 30-year gap, and in those 30 years, he successfully managed to cause the destruction of the Republic and the Jedi Order again, and turn a Skywalker to the Dark Side again.
There was no preventing involved. Palpatine was still actively shaping galactic events for all that time post-RotJ. RotJ’s victory was made completely hollow and illusory. Even the title “Return of the Jedi” doesn’t make sense anymore. The Jedi failed to return.
If the Sequel Trilogy had involved some Dark Side plot to resurrect Palpatine by pulling his spirit back from the netherworld of the Force, and our heroes had to prevent this and make sure Palpatine stayed dead, then that would count as stopping his return. But that’s not what happened. RotJ was just a tiny setback to Palpatine.
And of course, they didn’t prevent the Empire’s return, either. The Empire was back to full power, so there was nothing left to prevent.
I would have been fully aboard with the more optimistic ST that you’re describing. One where the heroes fight to protect and maintain the peace they had built as they’re confronted with resurgent threats. We could have seen how Luke’s Jedi Order and the New Republic are put to the test against many of the same problems faced by the Old Republic and Jedi, all while struggling to prevent a lapse back into authoritarianism. We could have seen the Skywalker family struggling to stay together amidst these events, and seen Luke and Leia working to keep the new generation from repeating the mistakes of the past and falling to the Dark Side. That’s a Sequel Trilogy I would have liked to see. Instead, all of that interesting story basically happened offscreen before TFA, with the heroes having failed at everything they set out to do.
What I mean is that they didn’t fully take over the galaxy. They stopped the Empire and Palpatine before they even got to complete their takeover. And no, the First Order doesn’t rule the galaxy in TLJ, if you’re thinking of “The First Order reigns”. The crawl mentions Snoke is sending his legions to seize military control of the galaxy, and Rey herself admits that their takeover is only complete within weeks, implying it isn’t finished.