So, the underlining theme of your desire to discuss John is to denounce the Eucharist?
John 6 is the classic Eucharistic passage which, along with Jesus’ words of consecration at the Last Supper (Matt 26:26-28, Mark 14:22-24, Luke 22:19-20), we point to to demonstrate the Eucharist in Scripture.
We know that Jesus was speaking literally is that the apostles believed and taught that he spoke literally (see 1 Cor. 10:16, 11:29). The same is true of the Christians of the first, second, third, and fourth centuries. Their writings show them to have understood and taught that Jesus’ words in John 6 were not symbolic.
We also know that Jesus was speaking literally, not figuratively, because to the Jews of his day what did “eating someone’s flesh and drinking his blood” mean?
Here is what Justin Martyr said of the Eucharist, and he lived closer to the time of the Apostles than you and me:
"And this food is called among us Εὐχαριστία [the Eucharist], of which no one is allowed to partake but the man who believes that the things which we teach are true, and who has been washed with the washing that is for the remission of sins, and unto regeneration, and who is so living as Christ has enjoined. For not as common bread and common drink do we receive these; but in like manner as Jesus Christ our Saviour, having been made flesh by the Word of God, had both flesh and blood for our salvation, so likewise have we been taught that the food which is blessed by the prayer of His word, and from which our blood and flesh by transmutation are nourished, is the flesh and blood of that Jesus who was made flesh. For the apostles, in the memoirs composed by them, which are called Gospels, have thus delivered unto us what was enjoined upon them; that Jesus took bread, and when He had given thanks, said, “This do ye in remembrance of Me, this is My body;” and that, after the same manner, having taken the cup and given thanks, He said, “This is My blood;” and gave it to them alone. Which the wicked devils have imitated in the mysteries of Mithras, commanding the same thing to be done. For, that bread and a cup of water are placed with certain incantations in the mystic rites of one who is being initiated, you either know or can learn.