Since you asked about the Book of Ecclesiastes,
a Midrash Yonatan (by Rabbi Yonatan Eybeschutz) came my way, and from it I saw something connected to another question you asked a long time ago about circumcision.

In Ecclesiastes it says: “Give a portion to seven and also to eight, for you do not know what evil shall be upon the earth.”
And in Midrash Ecclesiastes:
“Give a portion to seven” — these are the seven days of creation.
“And also to eight” — these are the eight days of circumcision.
“For you do not know what evil shall be upon the earth” — Rashi explains: if the Temple is destroyed and you will no longer offer sacrifices.

And Rabbi Yonatan Eybeschutz brought two reasons why the Holy One, blessed be He, did not create us circumcised:
1. So that the nations should not say, God forbid, that there is one authority for Israel and one authority for the nations.
2. Because the blood of circumcision atones like sacrifices.

There is more to elaborate on here, but this is not the place.

I don’t know how it’s possible not to see it clearly, unless you’re blind or biased. In any case, the Sages testify that they understood: “The sages sought to hide away the Book of Ecclesiastes because they found in it things that incline toward heresy” (Leviticus Rabbah 28:1). The Book of Ecclesiastes is perhaps the wisest and most honest book ever. The first postmodern book ever written. It belongs to wisdom literature and is an Epicurean graft, so its message is heretical. You can read this in Yaakov Malkin’s book The Garden of Epicurus, where he expands on this. Ernest Renan draws a psychological profile of the author of Ecclesiastes in his book I, Ecclesiastes. According to Renan, the message that emerges from Ecclesiastes is the philosophy of futility (for more, see his book). Ecclesiastes’ desperate efforts to reason things out are like the torment experienced by a great musician forced to perform a complex symphony with an orchestra that has not rehearsed properly. It is a paradoxical book that mixes sadness into joy and joy into sadness; it reaches no conclusions, but stages a discussion between pro and con. It loves life, while at the same time seeing its vanity. Above all, it never pretends. The idea of suicide, which for a moment passed through Job’s mind when he saw the wickedness in the world, never occurred to Ecclesiastes. How did the Sages put it into the canon? We are a strange people, as though created to display every form of contradiction.

A., why did that idea really never occur in Ecclesiastes?

By the way, there is a nice commentary on the book by Rabbi Yoel Bin Nun and Medan.

First of all, it’s pretty clear that Ecclesiastes was not written by one person, and there is no single idea there. There are ideas there, and arguments about them (probably written as notes on the text).

The spirit of the idea of suicide is indeed found there:
“And I hated life, because the work that is done under the sun was grievous to me; for all is vanity and a striving after wind.”

And the advice that repeats itself is:
“So I saw that there is nothing better than that a man should rejoice in his works, for that is his portion; for who shall bring him to see what shall be after him?”

And in this there is heresy:
“For who shall bring him to see what shall be after him?”

And the most important message in Ecclesiastes:
“For there are many things that increase vanity; what advantage is there to man? For who knows what is good for a man in life, the number of the days of his vain life, which he spends like a shadow? For who can tell a man what shall be after him under the sun?”

In two sentences he destroyed the entire fortress of conspiracy built by the charlatan advisers who determine and decide what is good for a person.

Why? “Utter vanity!” Ah! The proper condition for attaining life’s pleasures is defining them as vanities! He dismisses Job in a few words: “There is a vanity that is done upon the earth.” Ecclesiastes does not take life seriously. Ecclesiastes sees the futility of trying to reconcile divine justice with what happens in the world. A person has nothing to do but live peacefully, enjoy in moderation the wealth he earned honestly, and wait patiently for old age, which Ecclesiastes describes in friendly language. His pessimistic ideas comforted his tranquil conscience 🙂

It is no novelty that the Book of Ecclesiastes is a very deep book.
The questions the Sages raise in the Talmud and in the midrashim, they answer right there on the spot.
Likewise, the Talmud and the midrashim explain the Book of Ecclesiastes very well. And in all the commentators on the Hebrew Bible (Tanakh) you can find clear explanations.
Understanding the book requires depth.
Whoever longs to understand the Book of Ecclesiastes through outside literature, good for him. He just shouldn’t complain afterward about the nonsense they shoved into his brain.
Ecclesiastes an heretical book?? Not even a joke.

The Last Decisor — see Ibn Ezra’s commentary. Ecclesiastes is King Solomon!!!

Heretical in the strictest sense. The Sages and the commentators only ruined it. I do not believe that King Solomon wrote Ecclesiastes (if he even existed at all).

No doubt you always amaze us.
After going through the entire Talmud and the midrashim, Yalkut Me’am Lo’ez on Ecclesiastes, and the commentaries of Rashi, Metzudot, Radak, Malbim, and all the rest,
you discovered that the only one who cracked the secret of the Scroll of Ecclesiastes was Malkin the heretic, who sat in his garden and wrote a book for heretics???
Amazing as always!!!

Why, what is the Talmud, midrashim, Me’am Lo’ez, the commentaries of Rashi, Metzudot, Radak, Malbim, etc. to me? Cracking sunflower seeds. I’ve already moved on to electronic books. I lie there like a Saudi sheikh and scroll pages with my eyes on my smartphone, and you have no idea how many books I finish that way. It was the Torah I studied in comfort that stood by me. A stage will come when we can implant memories of learning without studying at all. To acquire all the learning experience of whomever we want.

Oh, and it’s not “garden” with a patach but with a segol — that’s your mistake. A brief history of Jewish heresy.