I keep forgetting that's only common parlance for the manga/anime/otaku community. The concept's obviously not original to them, but the word is and it's handier than, say, portal fantasy (which isn't a perfect fit for a lot of isekai anyway). You could probably call both Quag and Guardians portal fantasies just as accurately, but the metatext of characters being PCs within (or outside of) a game makes them seem more specifically isekai-ish to me.
Now, did you stumble across the
Truck-kun meme while looking up the definition of iekai?
Oh, and I thought of another few books.
There were at least two and possibly three novels based on the Castle Falkenstein RPG that do a decent job of feeling like they're something that could have happened in the game, helped by the fact that Tom Olam (one of the protags) is metatextually from our universe in the first place, with the RPG supposedly being written to introduce the concept of RPGs to the folks in the game setting and then ported back here later. From Prussia With Love and Masterminds of Falkenstein are both by
John DeChancie, and while they aren't quite up to his Skyway trilogy they're probably his second best work. Supposedly there's a third book (again by him) called League of Dragons but it may never have been printed - never saw it and even Amazon doesn't have it, but it left footprints on the internet so maybe it got written at least.
I'm honestly not sure if Nick Pollotta's Bureau 13 books count here or not, but they were explicitly based on Tri-Tac's Bureau 13: Tripping the Night Fantastic urban horror/comedy RPG, share a similar sense of quirky humor, and IIRC there are a few meta references that indicate that at least some characters are aware they're in someone else's fictional RPG. His writing on them is good enough to merit a look regardless of whether they're litRPG books or not - as is Illegal Aliens, an old TSR novel he co-wrote with Phil Foglio (of What's New? and Girl Genius fame). The guy does lightly comedic adventure stuff well when he feels like it.