How can one say that existence is good, without using Divine Revelation?

First you need to establish what is meant by "good". For its four letters, this is a large word!

Some parts of the definition of good may be supported empirically, whereas others are problematic.

Suitability to purpose. When God created the universe, he revealed part of His purpose for the things that he created. For example, the sun, moon and stars were created to establish the seasons and mark time. Thus some philosophers have looked to nature for examples that demonstrate purpose. The "Intelligent Design" movement falls in this category. A universe that came about by random chance and a universe that was designed with a firm sense of purpose should be distinguishable from one another.

Perfection in design. Nature has many creatures whose construction is nearly perfect. A Biblical worldview teaches that mankind fell and the corruption of our evil has tainted all of creation. This leaves us with systems in nature that are nearly perfect but have flaws. The argument then becomes, is our universe closer to perfection or imperfection? If impossibly close to perfection, then that is evidence of a good creator who created a good world. If otherwise, then random chance may have led to the current state of affairs. This line of reasoning inspired Darwin and others to search for "vestigial organs" that serve no purpose today but formerly did as proof of evolution. Thus creationists have shown how many organs once labeled as "vestigial" continue to serve a useful purpose; it just took time foir science to discover what that purpose is. This article speaks to the issue of vestigial organs: https://answersingenesis.org/human-body/vestigial-organs/vestigial-organs-evidence-for-evolution/

Generosity. Examples of symbiosis and altruism in nature are another line of reasoning. There are many symbiotic relationships among creatures.

Provision. How do all those creatures get enough to eat? Ecologists seem to have persuasive theories. Predator-prey relationships and their differential equations all seem tidy. However, when God answered Job from the whirlwind, He made a big deal about claiming that he regularly provides every material need for his many kinds of creatures. Some of those creatures their creator labeled as lacking wisdom, as if to say that their continued existence was a miracle dependent upon Him. I suspect that a deep analysis of the biosphere would show that more species should go extinct and more creatures die at earlier ages than they presently do. God would not have emphasized this if the survival of those creatures was so easy to arrange.

Objective morality. The existence of the conscience and substantial agreement between widely differing cultures as to what is right or wrong argue for a creator that has such qualities as well.

As a kind of back-up to the excellent answer given on 5th July, and the equally profound one given on the 6th July, I would point this sad fact out.

Consider how humanity uses, and abuses the word 'good'. Consider various human definitions of that word, then compare them with how God uses that word in what he has divinely revealed to humanity. The differences are truly shocking, and deliberately so, for our Creator will not tolerate sinful human thinking on that score. He has chosen to reveal to us what 'good' truly is, and that not one of us even gets close to divine good, for all humanity is sinful and we have no good within us whatsoever. Like a hammer-blow, the holy scriptures strike our thinking with full force, for until we realize how impossible it is for us to be 'good' as God defines 'good', our existence is futile.

Although the question refers to how it is through "Divine Revelation that we learn that creation/existence is good (Genesis 1:31)", the human corruption of God's good creation throws the divine spotlight on to human culpability for it becoming 'bad', despite remaining testimony to its divine beauty and purpose. Therefore, I list the following statements of the Father and the Son that can only be found in the divinely revealed holy scriptures:

Genesis 6:11 - "The earth also was corrupt before God, and the earth was filled with violence... for all flesh had corrupted his way upon the earth." Indeed, the perfect creation of Genesis 1:3 was "very good", but by the time of Noah, that had become past tense, despite the continuing beauty and wonder of creation.

Psalm 14:1-3 - "The fool hath said in his heart, 'There is no God'. They are corrupt, they have done abominable works, there is none that does good. The Lord looked down from heaven upon the children of men, to see if there were any that did understand and seek God. They are all gone aside, they are altogether become filthy; there is none that doeth good, no, not one." There is divine revelation that every sinner balks against and would complain about. Can anything good come out of that which is corrupt and filthy? 'No', is heaven's answer.

Romans 3:9-12 - "...we have before proved both Jews and Gentiles, that they are all under sin; As it is written, 'There is none righteous, no, not one'..." [Sounds familiar? The rest of that Psalm above is quoted plus reference to Micah 7:9.] Here is divine revelation as to what causes the corruption and filth of humanity - sin.

Romans 8:22-28 - "For we know that the whole creation groaneth and travaileth in pain together until now" yet it goes on to assure those who love God, who are called according to his purpose, that he works good towards those ones, and that all creation will yet be restored by him to its initial, perfect goodness.

2 Peter 3:12-13 - "Looking for and hasting unto the coming of the day of God, wherein the heavens being on fire shall be dissolved, and the elements shall melt with fervent heat. Nevertheless we, according to his promise, look for new heavens and a new earth, wherein dwelleth righteousness." That righteous re-creation will be utterly good, for the righteous judgment of God is utterly good.

Matthew 19:17 - Jesus said, "There is none good but one, God."

I suggest that last shocking divine revelation actually needs to be the starting point for us sinners beginning to understand all the revelations of God with regard to where we stand as to being 'good', or not. Once we are shocked to the core of our being at just how corrupt and sinful we are, in God's sight, then we might stop trying to justify ourselves as to having some meritorious 'goodness' about us. Then we might truly seek God and his only righteous means of entering into his goodness. That is why Jesus encouraged us to "Seek ye first the kingdom of God and his righteousness, and all these things shall be added unto you." (Matthew 6:33)

If we do not take seriously what God has already revealed to us by divine inspiration of the scriptures, we are never going to 'read' the 'right signals' about existence from the material creation, which is already in a state of corruption - due to our disregard for what God has already told us.

As 'good' can only be defined within divine revelation, dealing with God's definition is foundational to answering this question.

Since

1. according to Jesus, only God is good and,

2. according to Scripture, without the Father, Son, and Holy Spirit there would be no creation, and

3. there is divine revelation inherent in the creation, and

4. sinful man clearly sees the divine power and Godhead and refuses to glorify Him as God

the answer to the question "Could we also reach that same conclusion (with the same meaning of "good") with the light of natural reason?" has to be no for the simple reason that it is only with Divine help that we can reach that conclusion even with Divine Revelation. In other words, even though we have Divine Revelation the majority still reach a different conclusion.

The question and this answer assume it were possible for a creation within which there is no Divine Revelation to be found. This is impossible since the very fact of a reasoning creature is proof in and of itself.