I don’t think the destruction of the Starkiller is comparable to the Hosnian system, it’s clear that the Starkiller is not the First Order’s capitol, but rather just a really big weapon. TFA and TRoS paint it as Hux’s pet project, while Snoke is off somewhere else. If anything, by it’s sheer size, Snoke’s destroyer seems more like a big mobile base of operations from which Snoke leads the First Order (I mean, that’s where his throne is), which paints the FO as a mobile armada with no main headquarters. TRoS also has Kylo Ren command from his own destroyer as well, though Ascendant adds Coruscant in the background to show that after TLJ the FO is actually triying to take proper rule over the galaxy.
Your restructure of TFA also helped show that the Hosnian destruction is a way bigger deal, painting the assault on the Starkiller as a failure despite getting blown up by the end. So the opening of TLJ makes sense, even if it’s a bit hyperbolic. The First Order reigns, not because they’ve taken over the galaxy in the 5 minutes between TFA & TLJ, but because, having decimated the peaceful Republic, they are now the most powerful faction at play, being much bigger than Leia’s covert Resistance. The part about Snoke deploying his “merciless legions” is the only bit that could be interpreted as the FO having a much wider range than what we see, but I think that bit is just meant to set up Snoke’s surprise attack on the Resistance fleet, the “legions” being all the ships, fighters, walkers and troops that chase after them. Still, perhaps “merciless legions” could be changed to something less grand to avoid confusion, such as “relentless forces”.
The fleet that chases down the Resistance isn’t necessairly all of the First Order. And likewise, despite Holdo saying they’re “the last of the Resistance”, in TRoS we see there were a few others, such as Snap Wexley, that weren’t part of the group that escaped in TLJ. But like I mentioned in the redux ideas thread, the chase and battle in TLJ really feels like the last stand for both factions. The fact Snoke himself comes out of hiding to lead the attack and the overkill of throwing his massive ship and fleet and walkers after the Resistance is enough to suggest that he’s channeling most of his resources to make sure they’re wiped out.