Again, fine. What you just said (bolded by me) is again, the definition of a "sacrament": Something that symbolizes and is really what it symbolizes. But consider this (regarding the remainder of what I quoted above not in bold), no one in the Catholic Church says we "eat" or "consume" or in any way, with our jaws and teeth "digest" the Grace of God in Communion.
What we do believe is pretty much what St. Augustine teaches on THAT subject, to whit, that we RECEIVE the Grace of God spiritually (the "spiritual eating of Christ" as you call it), through the Eucharist (which is the Body of Christ), but it's not our teeth and jaws that somehow "extract" this Grace, but rather it's mysteriously given to us, to our souls. It is thus not "consumed" (as St. Augustine said). And of course this Grace isn't destroyed by digestion with teeth and jaws.
This (what I just wrote above) doesn't contradict anything you have quoted St. Augustine saying. In fact, it's reinforced by all the quotes you've posted tonight. And it's all Catholic teaching.
This is not possible for Augustine, because you say "through" the eating of the Eucharist. But Augustine says that we eat Christ without eating the Eucharist, we "eat" Christ "already," even before readying teeth and stomach. Otherwise Augustine would have said, "at the same time" or "through," instead of "already" at the moment of faith.