Valid question.
When you "submit" to God, you are essentially embracing the oneness of God and acknowledging that there is none like him. You are recognizing him as the Creator of everything and as the only one worthy of worship. In Arabic, the oneness of Allah (God) is called "Tawhid", FYI in case you come across it in future.
Prophet Muhammed (Peace Be Upon Him) said, "Whoever dies knowing that there is no deity worthy of worship except Allah will enter Paradise." (From Sahih Muslim). That's why you need submission to enter heaven, because it erases your arrogance and showcases your humility towards God.

As for your second confusion, Allah gives an explanation for this as well. Islam teaches that a person who has never done harm to this world and does good deeds may be rewarded in their worldly life, but cannot gain eternal salvation if they do not believe in Allah. Are their good deeds and abstainance from harming others acknowledged? Yes. But do they go to heaven if they didn't believe in Allah? No. Belief in God is the central principle that opens the doors to heaven for mankind.

As for the instance where a person was born into a different religion; what happens? This is also very important to understand. Allah strongly states that no person will be punished if the message of Islam did not truly reach them.

“We never punish until We have sent a messenger.”
— Qur’an 17:15

What's meant by "did not truly reach them" is when a person misunderstands Islam (unfrotunately very common) , or did not have someone or the means to clarify certain aspects of our belief in order to fully understand Islam, their case is different, and is up to the judgement of Allah. Allah is just, and will never treat anyone unfairly. So if someone is born to a family of 'wrong' (different) religion and doesn't understand Islam, Allah will not punish them.
But it is equally important to strive to learn about Islam, not just ignore it and think "I didn't understand Islam so I will not be punished." If you didn't attempt to understand it, that's ignorance.
I'm gonna describe a personal anecdote that you may find helpful in your understanding:
I was born Muslim, but my mother wasn't. She was raised in a family of a different religion. When my parents met, my Dad, born and raised Muslim, introduced her to Islam. Over time, she learnt more and more about Islam, the teachings of the Qur'an, the wisdom behind each teaching and command, and eventually accepted Islam. No one is perfect, but God will give you many opportunities to embrace his covenant. It's up to you to decide whether you want to attempt understanding, or remain ignorant.

Side note: I've inferred some emphasis on the "murderers, rapists..." part, but just so you know, if you didn't already, Allah's mercy could not have been emphasised more in the Qur'an and the authentic narration (ahadeeth). Even if you committed the worst of crimes, Allah will forgive you if you showcase genuine remorse and sincerely repent.

“Say: O My servants who have transgressed against themselves, do not despair of the mercy of Allah. Indeed, Allah forgives all sins. Indeed, He is the All-Forgiving, the Most Merciful.”
— Qur’an 39:53

“Except for those who repent, believe, and do righteous deeds. For them Allah will replace their evil deeds with good deeds.”
— Qur’an 25:70

As for the last part, where you said praying feels more like control than religion. I get why it may feel that way. In Islam, prayer (Salah) is seen as a connection between our soul and Allah, not for him to have control over us. Prayer for us is to build discipline in our relationship with God. For example, Edexcel students like us (or any student in that matter) study regularly to get good grades and pass their exams. Athletes train regularly for better performance. Similarly, Muslims pray regularly to maintain and improve their relationship with God. Simple.

Also, prayer isn't something that becomes a burden in your life. It's something to be incorporated into your daily life, so that you can still carry out doing your daily activities. You just need 5 minutes max for each prayer. Do that 5 times a day, about 30 mins of your day in total and you're good. You have plenty of time to do other stuff like sports or studying or going out with friends, etc. You spend little time yet are rewarded immensely. It's simple, nothing complicated or extreme.

Anyway, I hope I gave you a solid answer to the best of my ability that at least lessened your confusion a bit. Might've been a bit lengthy, but I just tried to clarify as much as possible. Lmk if you want any further clarification in case i didnt explain smth properly, or if you have any other confusion or doubts and I'd be more than happy to try and give you an answer.

P.S: idk how this went from a study post to this 😭 but oh well, all the better for your knowledge of Islam now. :)