Sending this to you specifically because I feel like you're the only normal brained person about this series.
I read Captive Prince a couple of years ago and liked it fine, as someone who doesn't have a strong preference for or against m/m and therefore isn't well-versed enough in its clichés to notice whether it reads like something full of cliché fanfic tropes. I really liked the use of language, some of the historical/political stuff, some of the ways that the characters are made complicated or unreliable.
The problem is, I've since coincidentally read most of the books that were Pacat's inspiration and. Well. Like 80-90% of the stuff in there has direct parallels in one of those. I'm not sure if there are more than three original thoughts in there.
I am not joking, if you read 1. The Lymond Chronicles and House of Niccolò by Dorothy Dunnett; 2. The Nightrunner series by Lynn Flewelling and 3. The Vampire Chronicles by Anne Rice, you will find that some of the scenes were ripped off beat for beat, many jokes were taken and just lazily reworded, a lot of the character beats and arcs are at best a mashup of 2 or maximum 3 things from those books with the serial numbers filed off really poorly. There's a scene where some characters discover a suicide victim whose structure and descriptions are completely ripped off from another series, except Pacat fails to see what made the original scene so impactful. A lot of the pretty use of language is also directly copied from those, including some really really specific descriptors. It's so blatant!
And I don't see how people are okay with this! I know people who are fans of several of these works and they're totally cool with it! And honestly if it were some rando's unpublished original project I'd be cool with it too, but as a published beloved worldwide phenomenon with rabid fans? C'mon.
And another thing is, all of the other series mentioned above are balls to the wall insane. She just... she just made it bland. She took off most of the edge and reshuffled the elements into a fairly straightforward MLM love story with some light kink thrown in. It's not a BAD series, but I feel like I'm disappointed that someone read all my favourite books and THAT was their takeaway?
I started reading, got to the name of the series and burst out laughing.
And, wow, you brought up something totally new that I hadn't thought about! I'm impressed. Genuinely. Usually, I rant about all the reasons I hate the series while debunking the standard anti talking points.
I've read the Vampire Chronicles, but that was in the 90s and I read Captive Prince in like 2016 or after. I had zero recollection of common points. I haven't read The Lymond Chronicles despite years of hurt/comfort fans bugging me to do so, and I think I got like a chapter into the Nightrunner series before getting distracted...
The reason I found Captive Prince annoying and derivative is that it also reads exactly like the original m/m that was available when it was first being written, most notably the work of P.L. Nunn who was extremely famous for horny fan art but who also wrote some original m/m fantasy novels. The scene where they finally bone reminds me heavily of the one from... uh... what's the P.L. Nunn one where the prince creeps on that archer dude and he gets raped because of course and then the prince has to be ~patient~ and it's peak boring 2000s rape recovery tropes?
The rape backstory is not only lifted from Fushigi Yuugi (the author's fandom at the time) but is shared with basically 100% of original m/m from that era. It's sometimes the uncle, sometimes the stepfather. One single time that I can think of, it was the foster brother, but mostly it's that "funny uncle" type. Unless it's gang rape from a bad Gundam Wing fic. That was also everywhere.
Once somebody told me the author had previously shipped Tamahome/Nakago, I realized that they'd taken what would be a kind of unusual anime ship and made it a thousand times more boring by dumbing down the scary, tall general with the tragic backstory into an uke-appropriate waif.
So I guess what I'm saying is that there may be some inspiration you aren't familiar with, but it's the same story as what you said above: this is the blander remix.
As for why people are okay with this... honestly, most of the people who really adore the series whom I've talked to point to how it encapsulates the exact tropes and tone they loved in anime fic circa 2000.
As a fellow weeb, I loathed the tone of said anime fic circa 2000 and could not wait for tastes to change, so Captive Prince is a hideous blast from an unlamented past for me.
I think most people who love the series aren't familiar with its inspirations. Those that are may have consumed them quite a while ago and don't realize quite how direct the parallels are. Or, for the inspirations that aren't overtly canon m/m, the lure of it being Exactly Right may overshadow other things.
(They also mostly haven't read 90s fantasy doorstops that actually contain political intrigue, tactics and strategy. Captive Prince is a piss poor entry into this genre and should be recced for m/m, not this other stuff people constantly think is in there. None of these characters can reason their way out of a paper bag.)
More than any of that, there's a lot of love left over from when the series was a serialized original story on Livejournal in an era when we had vanishingly few original m/m works, especially long ones in a fantasy alternate world.
I think that is what gets it a pass when a new m/m novel presented as "m/m romance" or posted on one of the webnovel sites would be mocked for unoriginality. Many Captive Prince fans aren't (or weren't when they read it) all that familiar with the smorgasbord of original m/m available today. Many are unfamiliar with anime fanfic circa 2000, so this feels adjacent to the fic they've read but a little fresher... instead of like the week-old sardine tin I found it to be.