And they both just sort of vanish partway through except for a few offhand mentions, to boot

When I gave one of my characters a pet dog the very good boy wasn't just there as set dressing; he actually played a role in the plot by leading police to his owner when he was in danger (dog got left behind, cop recognised the dog from a previous encounter and followed him). Later on he alerted everyone to a fire which helped them get out of the building in time. Such a good boy! I hope he got a new chew toy. ^_^

something that really jumped out at me on my recent read/commentary of that fic was that it introduces tons of characters and then a lot of them just end up disappearing from the narrative without their arcs being resolved at all, or get demoted to recurring background characters at best

Same with this book, you will have noticed. Like why even bother to have some of Kiragon's original colleagues survive if they never come back into the story? Or those others who go into cryo after she blows up the Extenuating Circumstances or whatever it was called? You can't just treat characters like set dressing; it's sloppy and insulting.

I mean, he had the Ring - but that comes with a whole laundry list of reasons why using it was a bad idea;). I was thinking less in terms of comparing the characters themselves and more in comparing the narrative perspective, admittedly.

Fair enough, but you can see how using that perspective doesn't fit when the protagonist is the biggest badass in the story rather than a hapless country boy who can barely hold a sword and has serious mental health issues thanks to carrying a cursed ring around with him.