It seems to me that you have an odd amount of facts. And by that I mean, your information seems to be scattered bits of disjointed facts that don't fit together. Judaism like most religions is best learned holistically, which is defined as: "the idea that things should be studied as a whole and not just as a sum of their parts. An example of holistic is health care that focuses on the health of the entire body and mind and not just parts of the body."
Your post has a lot of individual sources, but they seem to be lacking any cohesion that puts those sources together. Because of this, it seems like certain conclusions or questions that you have come to only exist because you haven't learned a holistic approach to Judaism.
Your first line
It is said that Hashem is the god of ALL people. But then how can it
be that many Jewish sages destined non-Jews for "hell"?
is just confusing to me and many other Jews.
An academic scholarly Jew would inform you that the Hebrew Bible doesn't really have a concept of hell and that Gehinnom is a Greek word. This illustrates that hell is not a core Judaic value but is a later development that takes hold in post Biblical Judaism. So some scholarly Jews are not convinced that hell is truly a Jewish idea but rather one that developed in the Babylonian exile or under Greek/Seleucid influence.
Many Jews who would identify as religious might take offense to your idea that a Jewish sage could "destine" anyone to hell. Many spiritually affiliated Jews would not agree with the idea that any Rabbi could send or determine who goes to hell. Only God has the power and the right to decide such things.
Your post further states:
It sounds like God hates Gentiles.
But I can certainly tell you every Jew I know would take offense to your idea that God would hate gentiles. Most Jews learn about Judaism in a more holistic way, and so we understand with absolute clarity that God loves gentiles. According to the Bible we are like God's firstborn son, but that doesn't mean in any shape or form that God doesn't love the rest of His children. Our being the firstborn is only because of the covenant we entered into with God, which came with a lot of costs. In contrast, God only asks the 7 Noahide commandments of His other children, whereas he expects us, His firstborn, to keep hundreds of commandments. God so loves His other children that their path to the life to come is much easier than ours. Whereas His expectations of us are so much more demanding, because we are like the firstborn, and because we agreed to do all the extra things He asked of us.
I know Rambam said that everyone who keeps the 7 mitzvahs has a share
in the world to come. But you never read anything about it, only since
This is an example of you having disjointed facts. The 7 Noahide laws is Talmudic in origin, based on very close interpretations of the Tanakh, which is why Rambam mentions them. The idea that Noahides have a share in the world to come is also Talmudic in origin, but like many things in the Talmud, only through a close analysis of the text do you understand this is what they're saying.
Joseph Albo, a Sephardic sage of the 15th century posits the afterlife to any pious gentile as being Talmudic in origin:
For this reason you will find that the Noachian and the Mosaic laws, though differing in matters of detail, as we shall see, agree in the general matters which come from the giver. They both existed at the same time. While the Mosaic law existed in Israel, all the other nations had the Noachian law, and the difference was due to geographical diversity, Palestine (i.e. "Eretz Israel") being different from the other lands, and to national diversity, due to difference in ancestry. And there is no doubt that the other nations attained human happiness through the Noachian law, since it is divine; though they could not reach the same degree of happiness as that attained by Israel through the Torah. The Rabbis say: "The pious men of the other nations have a share in the world to come". This shows that there may be two divine laws existing at the same time among different nations, and that each one leads those who live by it to attain human happiness; though there is a difference in the degree of happiness attainable by the two laws. This difference in the laws can not concern fundamental or derivative principles. Therefore the examination of the law itself is always of the same kind. But the examination relating to the messenger may undergo change. At all events the verification must be direct, though the verification of one religion may be different from that of another. The question whether a given divine law may change for the same people in the same land, we shall examine in the Third Book...
I believe Rabbi Albo comes to this impression from the following Talmudic debate
It is in accordance with the opinion of Rabbi Yehoshua, as it is taught in a baraita that Rabbi Eliezer says: It is written: “The wicked shall be turned back to the netherworld, all that nations that forget God” (Psalms 9:18). “The wicked shall be turned back to the netherworld”; these are the sinners of the Jewish people, as only the sinners are sentenced to the netherworld. “All the gentiles that forget God”; these are the sinners of the gentiles. From the fact that it is written: “All the gentiles,” it is apparent that none of the gentiles have a share in the World-to-Come. This is the statement of Rabbi Eliezer. Rabbi Yehoshua said to him: But is it stated in the verse that the sinners of the Jewish people will be like all of the gentiles? It is stated only: “All the gentiles that forget God.” Rather, the wicked shall be turned back to the netherworld, and who are they? They are all the gentiles that forget God. Gentiles who fear God do have a share in the World-to-Come.
As you can see, the Talmud is a collection of debate and arguments. If you were only to pick out one verse from this paragraph you might pick out: "it is apparent that none of the gentiles have a share in the World-to-Come." But doing so is incorrect. Because the entire argument was done to reach the final conclusion: "Gentiles who fear God do have a share in the World-to-Come. "
Since you were unaware that the idea of Gentiles going to world to come is Talmudic, highlights how disjointed your facts are. When you learn Judaism with an intellectually honest teacher they should strive to teach you the source of everything you learn so you would never err and think Rambam made up the idea that Gentiles go to the world to come. A teacher who shows you all the sources will fill in the needed context, and such context would make it hard to fathom the idea of God hating gentiles. Especially when you consider the following.
Out of the 3 major Abrahamic faiths, Judaism is the only one that allows non-believers into the afterlife. Most Christian communities believe only Christians make it to the afterlife, same for Muslims. And not only does Judaism allow non Jewish believers into the afterlife, based on the numbers there will be more gentiles in the afterlife than Jews. What Christian thinks there will be more non Christians than Christians in heaven? What Muslim believes that?
I think you should take a moment and go back to square one. Find yourself an intellectually honest teacher of Judaism. If you find such a teacher, then one day you will understand why your post and questions remind me of this proverb:
A single coin in an empty chest makes a loud and useless noise.
Because you will understand that disjointed facts can be like individual golden coins. Each coin is true and valuable, but if you only have a few such coins in a large empty chest then the sounds they make are loud and useless. But once you have more context facts that connect the other facts together then your chest becomes full.