Abrams really robbed us of [the wider galactic situation]. TLJ had no reason to expand on it. Any lazy writing lies with Abrams.
That’s quite a lot of blame to place on one guy. And sure, his was the cardinal sin of failing to properly establish the relationship between the First Order, the Republic, and the Resistance, but these were issues that could have been resolved in a single scene in TFA and which could have just as easily been resolved in TLJ. Instead we are given no explanation for the First Order’s power nor its resilience after their primary base and weapon was destroyed. We are given no explanation for what remains of the Republic, nor of why the Resistance consists of only a few ships at the start of TLJ. As you say, the Republic doesn’t exist in TLJ in any tangible sense, which means that the only real relationship which now exists for the audience is between the First Order and the Resistance.
The setup belongs at the start. ANH set up the civil war and the basic political structure. TPM setup the old republic and the trade federation conflict. ATOC setup the new separatist conflict. TFA setup the First Order and then dwelled on Luke for the rest of the film. Pretty much the focus was on Luke from the crawl to the end. A few lines would not have been out of place to help understand the galactic situation and how wiping out the senate would destroy the Republic. So yeah, I blame Abrams. An explanation doesn’t really belong in the middle chapter for something that happened in the first chapter.
I will reiterate, that Abrams did not destroy the Republic or turn the Resistance into rebels. If anything Abrams at least (not very successfully) tried to keep up the semblance of a different political situation with the FO residing on some ice rock in the unknown regions, and the Resistance the guard dog of the Republic, even though it was pretty obvious that the Empire vs rebels vibe should be maintained. Johnson threw out the window dressing and cemented the whole Empire vs rebels retread, that has once again become the central conflict, even though many other scenerios were possible based on Abrams setup.