(I was typing this on a computer that often lags and misses letters that I type, so forgive the occasional typo that I may have missed.)
I was PCA and never Lutheran, so I am not familiar with the nuances of the differences between standard Presbyterian models and Lutheran models. I'd read The Bondage of the Will many years ago, so I at least have a basic understanding of Luther's position, whether that is the standard Lutheran postition or not (although it seemed to perfectly jive with the Presbyterian/Reformed view).
As to your point about monergism/synergism and original sin, I will express my own observations about those doctrines and how they relate to Orthodoxy (so this is not necessarily the official Orthodox view, although in my own opinion I believe they accurately reflect the Orthodox view, but again, they are just my own thoughts).
With original sin/total depravity (while I realize that those two are not techincally the same thing, I will lump them together under the broader category of "the Reformed description of mankind's condition"), I believe that it is particularly Christologically problematic. The Reformed view of man and sin is often characterized by this adage: "I am not a sinner because I sin; I sin because I am a sinner." In other words, everything man does is sinful, and that is because it is man's very
nature to [only] sin, and it is that sinful nature which governs man's sinfulness. Just as a bird flies and eats worms because it possesses "bird nature", and just as a fish lives in water and breathes through gills because it possesses "fish nature", likewise man sins because man's nature is "sin nature". While Adam was created with one kind of nature that didn't drive him to necessarily sin, that nature changed after he fell to a different nature altogether, which is as different from originally-created human nature as it is from bird nature or fish nature.
The problem with this is that it describes sinning or not sinning as a function of nature. Christ, however, did not sin. According to the Reformed view, Christ did not possess fallen human nature. And since, in the Reformed view, fallen human nature and unfallen human nature are two completely separate and different natures, we are left with a scenario wherein God becomes "flesh", but not the same flesh that we have. He did not become what we are. He became a different kind of creature with a different kind of nature. As such, in that model, the Incarnation is destroyed.
Whatever we may say about man's sinfulness and why we sin and why Christ didn't sin, it cannot be because our nature changed from one kind to another. Human nature is human nature, and we and Christ possess the exact same human nature, otherwise there is no Incarnation, no Gospel, and no salvation.
As for Monergism vs Synergism, this also has serious Christological ramifications.
Orthodox theology, in pretty much all things, always starts with the Person of Christ. Who He is is the foundation of all of theology, and it is no different in the Orthodox aproach to anthropology and soteriology.
Christ is fully 100% God and fully 100% man, both natures being united together in one Person, without confusing or dividing those natures, and with neither nature adding to nor subtracting from the other. He is not zero-sum; he is not 50% God and 50% man. Each nature is complete and total, and His Person is fully both.
Christ Himself is our salvation. Our salvation is not simply what He did, as what He did is an outpouring of Who He is. He is our salvation, and He is
what our salvation fundamentally
is. As Christ is 100% God and 100% man, so too our salvation is 100% the work of God and 100% the work of man. Just as with Christ's Person, our salvation is not zero-sum. It is not 50% God and 50% man, or 99% God and 1% man, or even 100% God and 0% man, nor any other combination of numbers which add up to 100. Our salvation, like Christ, is 100% God and 100% man.
That is what true Synergism is, and that is what we mean when we speak of it. St Paul perfectly sums up Synergism when he says, "Work out your own salvation with fear and trembling [100% man], for it is God who works in you both to will and to do His good pleasure [100% God]."
However, this also works the other way around. Since what we say about Christ is what we say about salvation, what we say about salvation is what we say about Christ. If we say that we believe in a salvation that is 100% the working of God and 0% the working of man, then we are proclaiming a Christ who is 100% God and 0% man. That, too, destroys the Incarnation. It is the mirror image or shadow of Monophysitism, and it proclaims a Christ whose humanity is an illusion, or who never became man in the first place.
I hope that doesn't come across as overly polemical or confrontational. I simply wish to express why I believe those teachings are fundamentally wrong at their core.
My brother and his wife are Reformed. He and I were raised Lutheran. Actually as a WELS Lutheran I am very- monergistic? Is that the term? Somewhat similar to the Reformed in a way though there are differences between Calvinism and Lutheranism.
So yes, my understanding of original sin and the idea of synergism as in Orthodoxy and even the RCC are going to be a hurdle for me.