Kinda reads like you're just not really a fan of any approach to death that isn't ultimately tragic or sad (and stays that way)?
But even the KH2 examples you list are like immediately undercut in their own games? Axel dies never getting to see Roxas... only to then have a post-death conversation with Roxas where they both express hope that they have hearts and can be brought back. Xion gets to do a big monologue about her motivations, how she feels about Xemnas, how she feels about all her friends, etc and then dies with a smile... only to then to also get big hope swelling lines to Roxas post-death where she tells him not to be sad because she'll actually live on in Sora forever or whatever.
The series has always mixed tragic and hope. Bittersweet.
"There's always a light in the darkness" extends to the way the series handles death, too.
Maybe it's just personal tastes or interpretations, but I don't really think Xehanort's death is really resolved happily just because he's glad he isn't literally dying alone in his final moments after a lifetime of needlessly living completely alone and unloved with his life's only purpose failing? I think that's just the typical KH bittersweet grace note moment for him that every character gets in some way.
Man, idk. I think that flashback scene in his ending is like so sad. Also always really liked:
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