Have you been reading the patristic texts in their entirety (obviously I don’t mean everything all of them wrote, which would be a lifetime endeavor if it were possible at all, but whole texts or even whole lengthy passages), or simply looking for quotes dealing with this particular topic? I ask this because your post doesn’t make it entirely clear, and apologists on both sides will often take prooftexts out of context, providing a misleading impression.
In the case of Augustine, there are passages that sound as if he’s endorsing a “spiritual” view of the Eucharist, but then you find him telling people to adore Christ in the Eucharist. . .
Many of the Fathers simply speak of the Eucharist as a sacrifice without specifying just what they mean by it. But yes, my impression too is that they think of it in terms of a sacrifice of thanksgiving.
I don’t think this presents a problem for Catholic teaching at all. The Eucharist is a sacrifice of thanksgiving–but the one sacrifice of Jesus, for which we give thanks in the Eucharist, becomes present as we commemorate it. We aren’t holding a memorial service for something that happened in the past and which we remember with fond nostalgia. In the Eucharist, the sacrifice of Jesus breaks into our place and time through our act of “remembrance.”
Doctrine develops, remember. Once the question is posed: “what relationship does the Eucharist have to the one sacrifice of Jesus?” an answer that says, “it’s just giving thanks for the one sacrifice, but it is in no way to be identified with it,” is insufficient and thus heretical.
But the early Fathers weren’t asking that question. They were speaking in mystical terms and not defining the answers to questions that hadn’t been raised yet.
Edwin