You want to have it both ways but you cannot. If the sacrifice of Christ is on-going due to there being no time with God, as everything is one giant NOW, then it is irrelevant when Christ instituted the "sacrifice of the Eucharist," as if it this sacrifice does not have a terminus according to timelessness then it never had a beginning.
But if you condescend to operating with time as regards the time of an event of then you must submit the accomplishment of that event, that the atonement was finished by the death of Christ (Jn.19:30)
This recourse to switching to timelessness mode is compelled by the error of holding that last supper was instituting a perpetual sacrifice, but which it manifestly was not.
"This is my body" no more refers to bread literally being the Lord's actual or transubstantiated body than "This cup is the new testament in my blood" means the cup is literally blood, or David plainly declaring water was the blood of men and pouring it out on the ground as an offering to the Lord meant it literally was blood. Or that the Canaanites were bread for Israel.*
And rather than the Lord's Supper being "the source and summit of the Christian life," in which "the work of our redemption is accomplished," around which all else revolves, and engaging in this sacrificial offering being the primary function of NT pastors titled "hiereus"="priests," and being the essential to obtain spiritual and eternal life, in the life of the church the Lord's supper is only manifestly described in one epistle.
And in which, as hitherto described, the church as the body of Christ is the focus, which shows/declares the Lord's death for that body by showing care for each in taking part in that communal meal, the "feast of charity."
And nowhere is it set forth as the essential means of obtaining spiritual and eternal life, which is by believing the word of God, which words Jn. 6 tells us are spirit and life. Nor are NT pastors ever titled "hiereus" ("priests") except as part of the general priesthood of all believers. (1Pt. 2:9) Nor are they ever even shown distributing food, much less turning bread and wine into the Lord's body and blood, or charged with doing so as their primary or distinctive function.
Instead their primary or distinctive function is that of pray and preaching the word, which is what is said to "nourish" souls. (Acts 6:3,4; 1Tim 4:6)
Your premise “If the sacrifice of Christ is on-going” is incorrect. Jesus only dies one time. The Holy Sacrifice of the Mass transcends time. We are joined to that moment in time of His passion at Mass. However, our time on Earth marches forward.
You seem to have a lot of difficulty with this concept.
Here you say Jesus does not mean what He says:
“”This is my body” no more refers to bread literally being the Lord’s actual or transubstantiated body than “This cup is the new testament in my blood” means the cup is literally blood”
That’s too bad for you.