This isn't quite the same post as what I've been looking for, the one that theorized DSoD was Kaiba reinventing the Opening of the Mouth ceremony for a piece of jewelry, but it hits a lot of the same/similar beats
uh, the post I'm actually linking, is about the Grand Finale duel of the movie being intended as a "ceremony to summon Atem" which... is essentially the same concept as "reinventing the Opening of the Mouth" but not in so many words
took me an hour to go "wait I should say what I'm linking, not just what it isn't". so I should probably head to bed soon... braining isn't working now.
the core of both theories is "kaiba's deep-seated need to keep fighting and eventually beat atem has a direct link to set not getting to perform any rites for his king, or ever defeat him, the first time around" and i 100% vibe w that yeah
it's also a cathedral because that's where you get married. ohhhhhhhh