Waffle, you just perfectly illustrated why your system of...

Waffle, you just perfectly illustrated why your system of interpretation cannot stand. You keep saying you must use symbolism to “harmonise Scripture with reality,” but that reveals the real issue. You are not allowing Scripture to define reality. You are forcing Scripture to fit your worldview.

Once you decide the Bible must bend to human logic, science, philosophy or other religions, then of course everything becomes symbolic. At that point nothing in Scripture can speak for itself. You are not discovering deeper meaning. You are replacing the meaning.

You asked, “Who determines the Biblical boundary?”
The answer is simple. Scripture determines its own boundaries.
History reads as history.
Prophecy reads as prophecy.
Parables are introduced as parables.
Poetry is written in poetic style.
Symbolism is explained by the text itself.
This is why literal interpretation is the foundation, not because people are rigid, but because it is the only method that lets the text speak rather than the reader.

Your complaint about 47,000 sects proves the opposite of what you think. It does not prove that literal meaning is impossible. It proves that people ignore the literal meaning. It proves that people do exactly what you are doing, inserting their own interpretations rather than letting YHWH speak plainly.

Now let us address your example of the serpent in Eden. You say a literal snake is impossible, so it must be symbolic. But that is only your assumption. You have no Scriptural reason to deny the literal reading. The same book describes angels, cherubim and the voice of YHWH walking in the garden. If the supernatural Creator of the universe can create man from dust, speak the world into existence and breathe life into Adam, then enabling a literal serpent to speak is trivial.

Your argument is not based on Scripture. It is based on what you personally think is possible.

The deeper problem is this. Once you decide Genesis 3 is symbolic because it does not fit your logic, you now have to treat Romans 5 symbolically, because Paul ties the literal fall of one literal man to the literal redemption through one literal Man, Yeshua. If Adam is symbolic, Yeshua must also become symbolic at some point, because the entire structure stands or falls together.
You keep saying symbolism “harmonises Scripture with reality.”
But Scripture is the reality. Scripture defines truth.
Yeshua prayed, “Your word is truth.”
Truth is not defined by secular science or philosophy.
Truth is not defined by Islam or Buddhism.
Truth is not defined by your personal feeling of what is possible.
When Scripture records a miracle, it is literal.
When Scripture gives a parable, it announces it as a parable.
When Scripture gives a vision, it describes it as a vision.
When Scripture describes historical events, including the crucifixion and resurrection, it uses historical narrative.
If everything can become symbolic whenever the reader dislikes the literal meaning, then Scripture becomes meaningless.
This is why symbolism is not your tool for understanding.
It is your tool for avoiding the authority of the text.

Waffle, you always say you respect Yeshua, yet you reinterpret every clear claim He made. You reduce His words to metaphor whenever they contradict your philosophy. But you cannot claim to believe Yeshua while rejecting the way Yeshua Himself read Scripture. He treated Genesis as literal. He treated Adam and Eve as literal. He treated Noah, Abraham, Moses, Jonah and the prophets as literal. He treated miracles as literal. He treated His own resurrection as literal.

You are not following the method of Yeshua. You are following the method of the philosophers who came after Him.
The real issue is this.
If you think the serpent cannot literally speak, then you will think demons cannot literally exist.
If you think demons cannot exist, then Satan becomes metaphor.
If Satan becomes metaphor, then sin becomes psychology.
If sin becomes psychology, then the cross becomes symbolic.
If the cross becomes symbolic, then the resurrection becomes symbolic.
And with that, the entire Gospel collapses.
That is the end point of your method.

Waffle, Scripture does not need to harmonise with your worldview.

Your worldview needs to harmonise with Scripture.