As an aside, I really enjoy the "cosmology" of your setting. I think I've mentioned that before, but still. Interesting hints and notes of various real-world ideas about multipart / multilayer souls, that fit together as a nice alternate foundation for the quirks of D&D's magic, etc.
Spoiler: Digression/Tangent
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As a sort of contrast/compare, in one of the WIP worlds -- which sounds more similar than I realized as I type it out here -- what makes a living thing alive is a "spark" (to use your word) of light from a sort of universal "pool" of divine/bright/solar energy (the original deities were "star gods"), which is also where most magic comes from, but few mortals can directly tap that energy without a god or spirit's help, outside of a few narrow traditions specific to various Peoples. Aging and death are generally ascribed to some variation on the idea that the mortal flesh cannot forever contain that impossible brightness, such as it burns up the body slowly from the inside or all light is drawn back to the source so it won't stay contained or the like. Reclusive mystics and theosophers are believed to extend their lives as part of their mastery of their own bright soul and the flow of energy, and legend has it that the Sun People, who were wiped out with their empire at the end of the "bright age", were immortal via such mastery.
Bolstering this belief... there's another way to be immortal, or at least ageless: don't have a "spark" in the first place. There are beings -- dark spirits, certain monsters, the Twilight People -- who are given life by a dark mote of the infinite dark/cold/void/khaos that came before.
And yes, within the setting, there are those humans who consider anything "lives" without a spark to be an abomination, a walking talking breathing affront against the gods and nature.
Of note to the thread, real dragons, not just big mean lizards, but actual dragons, are also possessed of the dark mote, rather than the bright spark. They are creatures of dark flame and smouldering shadow.