I was also a little fuzzy on this, but
a post I saw on Reddit made a really good observation: the term that they commonly use to describe the energy used to sustain the Endless in English is 'life force', but in Japanese they use 'seimei-ryoku', which is the same term that was previously translated as 'corporeal aether' (the type that sustains your physical presence, seperate from your soul - it gets spent by exerting yourself, is replenished via eating/drinking, and without it you'd become a ghost). 'Life force' seems just be what non-scholarly people call the same concept, from what I can gather.
So I think that Sphene's plan was basically to turn the physical bodies of the entire population of the Source into a giant aether bonfire to power the Endless (and then possibly convert the leftover souls into cells + Endless, which would require more life force, which would require absorbing more reflections, etc...). Grim!