Aramaic is hardly central to the argument, since Greek also has words that could have been used to indicate Jesus’ words at the Last Supper were merely symbolic.
Exactly. That's why I don't get the force of an argument from Aramaic.
...since Greek also has words that could have been used to indicate Jesus’ words at the Last Supper were merely symbolic.
No one says that Jesus' words were "merely symbolic." It was not Jesus' words that were the symbols. Aside from that, everyone concedes that "is", is the substantive verb used. No one disputes it. If you want to adhere to the strict meaning of the words in this particular instance, (I would expect the same consistency elsewhere) so as to exclude by some grammatical rule a metonymical understanding of the expression, then I can't think of anything more alien from the strict meaing of the term is, than using it for being converted into something else, which, as far as I can tell is unknown to Greek or Aramaic, not to mention any other language.
As to the symbols themselves, of bread and wine, I agree with Augustine, et al, that the expression that Jesus used here, which is uniformly used in Scripture when the sacred mysteries are spoken of, is metonymical; iow, the name of the visible sign is given to the thing signified. Augustine says,
"“Had not the sacraments a certain resemblance to the things of which they are sacraments, they would not be sacraments at all. And from this resemblance, they generally have the names of the things themselves. This, as the sacrament of the body of Christ, is, after a certain manner, the body of Christ, and the sacrament of Christ is the blood of Christ; so the sacrament of faith is faith” (August. Ep. 23, ad Bonifac.)
"The Lord hesitated not to say, This is my body, when he gave the sign" (Cont. Adimant. Manich. cap. 12).
"Wonderful was the patience of Christ in admitting Judas to the feast, in which he committed and delivered to the disciples the symbol of his body and blood” (August. in. Ps. 3).
[emphasis mine}
Cordially,