If Zodiark was able to be powered through assorted organic matter, why didn't the Amaurotians feed Him an equivalent amount of shrubbery and small animals in the first place to achieve their goals, rather than sacrificing themselves to do it? It also raises some interesting philosophical questions with the Ea and their discovery of the second law of thermodynamics. If Zodiark doesn't operate on a law of equivalent exchange, then the Ancients have effectively created a perpetual motion machine and proven the Ea wrong. Sacrifice 5 Amaurotian souls for energy, activate miracle of your choosing, and then sacrifice 5 shrubs to return the Amaurotian souls from the graveyard back to your hand. Wish for more shrubs when you run out. Repeat endlessly until all your wishes are fulfilled and you win the game. Sounds like an idea for a blue/black deck design. I'd be on board with that plan if it were so simple, except that it would make for a boring story.
I remember an interesting discussion with Veloran a couple of months ago about Sanderson's laws as they relate to magic systems. A lot of what the Ancients do with Creation magic is, at present, soft magic. One of the critical insights from the lecture was that it's important to have a victory condition established in advance. Even with soft systems, you can't just snap your fingers and solve the problem for free, or else you've simply sidestepped the conflict.
This is also part of the reason why Shadowbringers outlines the centuries of hard work, cooperation, and sacrifice involved in allowing G'raha to alter the past, and even then it was off by a few centuries. That's also presumably why the Alexander storyline emphasized that Alexander consumed large amounts of energy with every time leap and would have drained the planet dry had it not sealed itself off into a single timeless instant. If time travel ever becomes so simple and trivial that you can alter past events without effort and consequence, then we can arbitrarily go back at our leisure and undo any event that we don't like. Character deaths are meaningless because we know that we can always go back in time and resurrect them at will. As attached as you may be to the people on Elpis, it's really not good for the future direction of the story, and the writers would do well to not cave into that.