I think that the 4th place in a regional Olympiad is a very good EC to mention. Your extracurricular activities do not need to be academic and do not need to have anything at all to do with your high school. They also do not need to have anything at all to do with your intended major. A part time job is an extracurricular activity. Volunteering is an extracurricular activity. Looking after younger siblings is an extracurricular activity if you need to do this on an ongoing regular basis.
I would expect that the admissions staff at the universities that you apply to will include someone who understands the grading scale in your country. I personally have no idea how to interpret a 16.3 on a scale of 20, but someone will know at the schools that you apply to.
Whether applying test optional will impact merit aid will most likely depend upon which universities you apply to.
What is your budget? What is your budget if you do not take out any debt at all?
Will getting a bachelor’s degree in the US help your chances to get a job in Morocco after graduating? Do not expect to be able to stay in the US after you get your bachelor’s degree.
Good luck with this and best wishes!