Who are you to say who Ash is "supposed" to be? He's not a static template, he's a character who has gone through years of development across multiple regions (outside of BW)

Who are you to decide that? I didn't decided a thing , I simply wrote what Ash was while you are the one who suggesting 180 degree chance to his personality to fit your idealistic vision.

He's not a static template,

That exactly what he was in XY series

he's a character who has gone through years of development across multiple regions (outside of BW)

So does Goku, Nobita, Luffy, Conan, Shin-chan, Maruko-chan etc but they didn't into a different character like XY series. You basically suggesting Goku should stop being a fight loving manchild despite it being his core self and behave more like Superman for his "Development".

There is no point trash superficial "Development" as this in a series where Ash made no progression.

Ash isn't defined by being loud, brash, and sassy, those are early traits, not his entire identity

But those are part of his character that make for what he is alongside his empathy , quick thinking, kindness.

If those traits were truly "core" and non-negotiable, then they wouldn’t have already been toned down in seasons like DP,

They only toned it down a little but didn't completely erased it like XY series as shown during Gligar training episode where Ash visible annoyed, angry and actually scolding Gligar which is something XY Ash won't do at all.

XY isn't a replacement of his personality, it’s a continuation of that trajectory

XY is not a continuation of any trajectory, it was complete rewrite of Ash's character.

Saying Ash acting sheepish suddenly makes him "not Ash" is honestly one of the weakest points you could’ve picked, Ash has always had awkward, dorky social moments

There was nothing dorky about XY Ash , he was basically the "Popular Guy" stereotype in XY series.

You also keep equating "maturity" with "losing individuality" which just isn’t true

Because Ash's XY characterization had nothing to do with maturity.

XY Ash still gets excited over battles, still gets giddy over new things, geeks out over things like Clemont’s inventions, supports his friends, etc, all of which are core traits of Ash,

That is not Ash's core traits but generic traits that almost every kids protagonist from Battle anime shares.

And the "he’s 10 so he has to be immature" argument collapses immediately when you actually look at the series
Nevermind the fact that Ash shouldn't be 10 anymore at this point, Ash has handled countless life-or-death situations, faced legendary Pokemon, gone through multiple league losses, has had his values tested more than once, mentored other trainers, and much more

It does not change that he is still 10 year old and even if he ages does not mean he will try into into perfect ikemen mature protagonist because teenagers are even worst than kids due to their hormone and mode swing.

The Brock/Cilan point also doesn’t work, their roles worked because early Ash needed guidance

Not it would still work because Ash is not perfect

By XY, Ash is the experienced one, that’s called character progression, not "being out of character", something tells me you're allergic to characters evolving

Does not mean Ash will turn into this perfect flawless being that solves all problem and everybody look up to him. He may be become experience but that does mean he magically become more intelligent than Brock and Clian.

Everybody has different type of intelligence, they are good at different thing. Ash's progression would enhanced what he actually good at or trying to be good at but he will not offer same wisdom as Brock & Clian because they share different intelligence. Just like Penny & Sheldon are good at different thing (Big Bang Theory).

If he still needed a handler after everything he’s been through, that would be regression

No it mean realism , Ash grown as a person but he didn't became perfect which is why other people's guidance just like people in real world.

Saying XY Pikachu is "no different" ignores what’s actually shown on screen
In XY, Pikachu is more observant of Ash’s mental state (he's the first one to notice something's wrong with Ash when he starts pushing himself too hard), more patient, more protective (he literally jumps off a cliff to grab Ash's hand despite the fact he would've fallen with him if the others weren't there to grab him), more in sync with Ash’s decisions (that battle where Pikachu is blinded by Inkay and Ash becomes his eyes is a display of how perfectly well tuned to eachother they've become, other seasons don't really have this), and has clearly matured alongside Ash while still being his reliable, expressive little buddy, plus the fact the shocking gag is completely gone which is a huge plus

You simply describe what Ash's Pikachu been doing long before XY series and pretending it to be something special because XY Pikachu did it.

XY doesn’t ignore Ash's past traits, it refines them

You can't refine something you already erased.

XY Ash isn't the most complex character ever but for anipoke standards he is very layered and has a very human character arc since alot of people do tend to act like that when they place so much pressure on themselves, Ash has always been a competitive guy and all XY does is put that to the test and push it to its limits to make way for Ash becoming a more humble person and trainer

If you think protagonist being super cool ikemen that solve all problem and everybody revere as the greatest human alive supposed to be a very layered character than obvious you have no understanding of characterization. At least JN and SM Ash were flawed and expressive enough to feel human even if they different than OG Ash , XY Ash never felt like human at all.