The Catholic response is in BLUE
You support the notion that the Eucharist is merely symbolic - there are several reasons why Catholics disagree:
You've suggested that because Jesus also said he was "the door", "the vine", etc., John 6 should be considered in the same way. First, metaphors of doors and vines work because Jesus is LIKE a door and a vine, etc. Through Him (door) we can enter the kingdom of heaven and He is our spiritual vine. However, Jesus goes far beyond a metaphor by saying, "My flesh is TRUE food, my blood is TRUE drink" (Jn 6:55).
Furthermore, Jesus made no attempt to explain to his disciples that he was only speaking metaphorically, "After this, many of his disciples drew back and no longer went about with him" (John 6:66).
Finally, the figurative usage of "eat the flesh and drink the blood" means to inflict serious harm upon a person, either physically or through slander (Micah 3:3, Psalm 27:2, Is 9:18). So, appropriate metaphorical usage of this phrase would mean that Jesus was suggesting that everlasting life would come to those who hate or revile Him?? This doesn't make sense at all.
Paul agrees with the literal interpretation: "The cup of blessing which we bless, is it not a participation in the blood of Christ? The bread which we break, is it not a participation in the body of Christ?" (1 Cor.10:16). He goes on to talk about receiving the Eucharist unworthily - if it is merely symbolic, why is this such an offense?
I'm very glad you accept Cyril's writing in support of your presuppositions, so here's what he had to say about the Eucharist: "The bread and the wine of the Eucharist before the holy invocation of the adorable Trinity were simple bread and wine, but the invocation having been made, the bread becomes the body of Christ and the wine the blood of Christ" (Catechetical Lectures 19:7)
and
"Do not, therefore, regard the bread and wine as simply that; for they are, according to the Master's declaration, the body and blood of Christ. Even though the senses suggest to you the other, let faith make you firm. Do not judge in this matter by taste, but be fully assured by the faith, not doubting that you have been deemed worthy of the body and blood of Christ. .. . [Since you are] fully convinced that the apparent bread is not bread, even though it is sensible to the taste, but the body of Christ, and that the apparent wine is not wine, even though the taste would have it so, ... partake of that bread as something spiritual, and put a cheerful face on your soul" (ibid., 22:6,9).**
Indeed, yet He is speaking of a true communion with HIM!!!
** How much of a more true communion can one have than with the literal interpetation? " I solemnly assure you that unless you eat the flesh of the Son of Man and drink his blood, you do not have life within you. Whoever eats my flesh and drinks my blood has eternal life, and I will raise him up on the last day. For my flesh is real food and my blood is true drink. Whoever eats my flesh and drinks my blood remains in me and I in him" (John 6:53-55). **
Jesus is attempting to have them see that He is an infinitely superior source of sustenance; indeed food and drink that leads to eternal life, not just the nourishing of this temporary human body.
** I agree completely, and this is exactly consistent with the literal interpretation. Some of his disciples said, "This is a hard saying. Who can accept it?" (Jn 6:60). Jesus replied, "Does this shock you?" (Jn 6:61), and he allowed those who couldn't accept this teaching to leave him. He didn't call them back and tell them they had misunderstood him, which he had done every single other time His followers didn't understand (Matt. 13:36-43, Matt. 16:5-12, Mark 8:14-21). By the way, John 6 is the only example in the Bible of disciples leaving the Lord over a doctrinal issue. **
Just as eating meat in pagan worship is sharing in the worship so too taking the Lord's Supper is sharing in worship of Him.
** The fact that taking the Lord's Supper as you call it is sharing in worship of God is quite true and irrelevant to your objection, as this says nothing against the literal interpretation. Furthermore, Paul writes, "Everyone is to recollect himself before eating this bread and drinking this cup; because a person who eats and drinks WITHOUT RECOGNIZING THE BODY is eating and drinking his own condemnation." (1 Corinthians 11:23-29 NJB). Paul very clearly felt that this activity was much more important than a mere symbolic gesture. By the way, you also neglected to respond to the true symbolic meaning of the phrase "eat my flesh", which is to "cause serious harm". This is yet another reason why the literal interpretation works better.
Peace be with you