Imagine a religion like christianity is totally of human origin, what would we expect to find?
Lol, I would expect that by now, we would found direct evidence of its fabrication.
Take the one thing that would proof for sure that Christianity was made up by the disciples. Just produce the dead body of Jesus and there you have it, Christianity disproven. And the Jews and Romans had ample opportunities?
Lets say we don't have the ability to do that, the next thing is to look at the disciples. At any point under trial, they could have admitted the whole thing was made up, but instead they were killed. Either they were insanely stupid or they really believed in what they saw was the risen Jesus.
Take for instance a skeptic who is actively killing Christians, then suddenly all of a sudden is promoting the message of the cross. What happened there?
On a more serious note, lets assess your criteria. I've numbered them.
1. Only a handful of people get to witness these alleged 'signs'
Firstly notice, I have replaced the word 'miracles' with a substitute. In fact the word 'miracle' (or 'miraculum') never appears in the gospel account, rather than language that John (and the other writers of the NT) uses is that of 'signs' and 'wonders' rather this modern idea of miracles.
This introduces an important but subtle differences that signs and wonders are displays of power and goodness; rather than the stuff of legends or mythos (which is what connotation the word miracle carries in pop culture). They are intended to confirm the truthfulness of the message of the one performing it.
This means to assess the truthfulness of a religion we have to assess the contents of the message (aka. what does the religion say about this world, about humanity, about morality), since the miracles (at least in Christianity) are acted to verify the authenticity or identity (or creds) of the speaker.
So its actually back-to-front. The aptly named signs were the verification back in the day for the message, and nowadays we have the preserved written testimony. John writes about this matter:
" Jesus performed many other signs in the presence of his disciples, which are not recorded in this book. But these are written that you may believe that Jesus is the Messiah, the Son of God, and that by believing you may have life in his name."
In terms of numbers, what you actually have is at least 513 people witnessing the resurrection of Christ, 900 men (not including women) witnessing 2 occasions of Jesus multiplying loaves and fishes etc; plus several others miracles with differing.
2. There are never any prophets ever again when these acts could be independently recorded and verified
3. There are no supernatural methods [today] employed for god to make his existence known.
These are basically saying the same thing. If we cannot accept the testimony of those who witnessed such signs then yes we would have no 'supernatural methods'.
To answer your question as to why not more today, a different writer writes:
"In the past God spoke to our ancestors through the prophets at many times and in various ways, but in these last days he has spoken to us by his Son, whom he appointed heir of all things, and through whom also he made the universe. The Son is the radiance of God’s glory and the exact representation of his being, sustaining all things by his powerful word. After he had provided purification for sins, he sat down at the right hand of the Majesty in heaven."
The reason why God is not sending any more prophets (again another reason why I reject Islam and Mormonism) is because his will/plan are finished in Christ, and there is no new words/message. I've often used this analogy, before a film premieres we are given like short clips e.g. trailers that reveal part of the movie; and then once the movie is released we don't go back to trailers, we watch the movie.
There is no need for miracles today to confirm the testimony of the prophets is because Christ is the final 'prophet'. However his work is carried out today in the church. As his people share the message and people's lives are changed - that is the wonderous acts of God that are continuing to this day (so the nature and extent of his works have changed).
This authority or verification then is carried on, in the preservation of the eyewitness' testimony, Peter writes close to his death:
"So I will always remind you of these things, even though you know them and are firmly established in the truth you now have. I think it is right to refresh your memory as long as I live in the tent of this body, because I know that I will soon put it aside, as our Lord Jesus Christ has made clear to me. And I will make every effort to see that after my departure* you will always be able to remember these things.
For we did not follow cleverly devised stories [myths] when we told you about the coming of our Lord Jesus Christ in power, but we were eyewitnesses of his majesty. He received honor and glory from God the Father when the voice came to him from the Majestic Glory, saying, “This is my Son, whom I love; with him I am well pleased.” We ourselves heard this voice that came from heaven when we were with him on the sacred mountain. We also have the prophetic message as something completely reliable, and you will do well to pay attention to it, as to a light shining in a dark place, until the day dawns and the morning star rises in your hearts. Above all, you must understand that no prophecy of Scripture came about by the prophet’s own interpretation of things. For prophecy never had its origin in the human will, but prophets, though human, spoke from God as they were carried along by the Holy Spirit."
*e.g. by writing this letter.
3. There are no supernatural methods [today] employed for god to make his existence known.
4. He is equally constrained in communication method (written eye witness testimony) as mortal humans are
Lol maybe God knew that naturalists would just reject any supernatural methods or explain anyway everything naturalistically.
In all seriousness, God has primarily utilized the spoken and written word about himself as his revelation rather than miraculous.
Secondly, God has revealed himself in such a way that most people think is frustrating or even foolish, Paul writes:
"For since in the wisdom of God the world through its wisdom did not know him, God was pleased through the foolishness of what was preached to save those who believe. Jews demand signs and Greeks look for wisdom, but we preach Christ crucified: a stumbling block to Jews and foolishness to Gentiles, but to those whom God has called, both Jews and Greeks, Christ the power of God and the wisdom of God "
and it is this same message that Christians proclaim today.
Jesus prayed this prayer "I pray also for those who will believe in me through their message, that all of them may be one, Father, just as you are in me and I am in you. May they also be in us so that the world may believe that you have sent me".
The point is that people don't believe on the basis of signs and wonders, but rather on the acceptance of spoken (and written) testimony.
Knowing a person has always been to do with speaking. I don't know a person if they don't speak to them. God is the same (and we inherit that aspect from him).
"He said to him, ‘If they do not listen to Moses and the Prophets, they will not be convinced even if someone rises from the dead.’"
5. People believing in the religion is what matters (not helping others or something that actually aligns with the sense of morality that humans are imbued with)
6. Belief is enforced under threat of punishment in the after life.
Even religion or ideology demands commitment, and what that entails differs. Islam for instance requires certain good deeds etc.
What you are tapping into is the question of "why does God send supposedly good people to hell?"
Because if God says it is not good to reject him, then you are not good.
In the case of Christianity, people are not saved because they are good, but because they trust in Christ who was good.
But belief and actions are not completely inseparable. The idea of 'faith' is closer to idea of trust than mere intellectual acceptance of the existence of God. And when you believe something it affects your actions. If you believe that God is good and loving, then you would be good and loving because God is; not merely out of self-preservation.
John writes: "From this time many of his disciples turned back and no longer followed him. “You do not want to leave too, do you?” Jesus asked the Twelve. Simon Peter answered him, “Lord, to whom shall we go? You have the words of eternal life."
The reason why people believe something is because they believe the testimony of those who saw, heard things firsthand.
In most cases, people stay Christians not out of fear of self-preservation against judgement, but rather they actually love Christ for who he is and what he is done, as John writes in his first letter:
"Dear friends, since God so loved us, we also ought to love one another. No one has ever seen God; but if we love one another, God lives in us and his love is made complete in us.... God is love. Whoever lives in love lives in God, and God in them. This is how love is made complete among us so that we will have confidence on the day of judgment: In this world we are like Jesus. There is no fear in love. But perfect love drives out fear, because fear has to do with punishment. The one who fears is not made perfect in love. We love because he first loved us. Whoever claims to love God yet hates a brother or sister is a liar. For whoever does not love their brother and sister, whom they have seen, cannot love God, whom they have not seen. And he has given us this command: Anyone who loves God must also love their brother and sister."