If everything was predestined by God, it would mean humans have absolutely no control over their actions. God would then not be able to judge humans for their actions as they were not the ones who caused them to occur. If He did judge them, He would be considered unjust, which is impossible. It cannot be said that He cannot judge humans, because if that was the case then no reward or punishment can be attributed to them. At the same time, there would be no sense of responsibility for their actions in this world. An oppressor, for example, would simply blame God for his wrongdoing. If everything was predestined life would have no purpose, the hereafter would be unnecessary, and morality would not even need to be a topic of discussion.
If everything is not predestined, it could be argued that humans have the totality of free will. If one has the totality of free will, everything one does is entirely in their control. Therefore, the day one is born, the family one is born into, the day one will die…etc. would all be in our control as well. However, this is not possible. God is the most Powerful and nothing can go beyond His control, even our free will. As much as God does not fully predetermine our future, our free will has its limitations as well. Therefore, if there is neither totality of free will, nor complete predestination, there must be something in between.
In Islam there is a balance between free will and predestination. It is important to understand that there is no contradiction between these two. We, as the servants of God accept our responsibility over our actions, while remembering that He is the Ruler over the universe.
In his book, Justice of God (2019), Ayatullah Naser Makarem Shirazi explains this concept with an example of an electric train.

Imagine there is an electric train, which has an electric cable passing along the side of the train. If the electric cable were to stop conducting electricity, or disconnect from the train for even a second, the train would stop moving. The driver, however, can direct the train and change its speed as he wishes. But if the electricity stops, even the freedom of the driver to maneuver the train stops.
In this example we see that the driver is free to maneuver the train as he wishes. However, his freedom is limited by the train’s dependence on the electric cable. Similarly, human beings have the freedom to make decisions, but everything in the end is limited by our dependence on God. If we were to be disconnected from God for even a moment, we would have no ability to do anything, because God is the overall Ruler of the universe who through His Mercy has given us the free will.
References
Makarem Shirazi, N. (2019, December 14). Justice of God. Retrieved from https://www.al-islam.org/justice-god-naser-makarem-shirazi
Thank you for making an effort to answer those difficult questions. However, I don’t think that this article answers some essential issues regarding free will and predestination. I think this post on Reddit is the best answer I have seen on this question: https://www.reddit.com/r/MuslimLounge/comments/jy2owg/do_we_have_free_will_or_has_god_predetermined/
A great website otherwise. All the best to you.
Thank you for your feedback!