HI Saddam! Here are some notes:
his or her (fluency) --> English is always changing, especially to follow the changes in society and social expectations and social norms. Now, you can just say they/them/their - this has also been used for a long time when we don't know who the person is exactly. So:
- someone can set their life according to their choice.....
due to which one can definitely be happy... (?fluency) --> this phrase doesn't make sense, sorry. What do you mean? Maybe: Also, with a good income, someone can set their life according to their choice, based on what makes them happy./according to the way they want to live.
Let me know if you have any questions.
Cheers!
Kae