In Star Wars we didn’t have any background (yet) and as you say it establishes the starting power balance pretty well even in the first scene, so it’s a fine jumping off point where it doesn’t matter if we have to fill in the blanks for ourselves. Here we know how Return of the Jedi ended, and we aren’t given much explanation of how we got from there to here.
I don’t think they’d have to do much; to make a comparison I think the background we get for Kylo Ren and the hints of his story we see with Luke in The Last Jedi was just enough to set the stage and make it work. Similar hints of what happened to the Republic and a little origin for the First Order would have been enough to sell it.
Also doesn’t help that 2/3 characters we get to know in the First Order act like incompetent angsty teenagers. I don’t mind Hux or Kylo Ren, they’re both entertaining characters (though I wish Gleeson would’ve chewed the scenery just a little less), but I think it would’ve helped taking the First Order seriously if we could’ve had some grownup character that wasn’t a cackling evil enigma (and who is now dead).
Yeah, the line of logic points straight to Finn and Rose and by extension Poe (who should also be able to figure out roughly what happened even before they reunite), so I don’t buy the idea that they’d be able to keep anything secret and Poe still comes across as a callous dick.
Anyway, niggles and negativity aside, now that I’ve had a day to digest it I think I can safely say I liked this one more than TFA, mostly because of how they developed the characters. Luke’s story was really good (and Mark Hamill did an excellent job) and all the interactions between him, Rey and Kylo Ren were really satisfying. To compare, it was fun to see Han Solo again in TFA, but he was basically just an aged Han Solo and there wasn’t much about him that was all that interesting. Luke by contrast has changed so much, and you could feel the weight of responsibility on him every moment he was on screen.
TFA was much slicker where this was a little too scattershot and unfocused at times. Finding a way to keep it focused on the two main threads (Jedi Island with Luke and Rey and the pursuit of the rebels with the other characters) and cutting out the trip to Canto Bight would’ve helped pacing and kept the pressure going in the chase. I think it made up for it with the things it did well, though.