Hi! Well done for asking about extracurriculars! They are very very very important in US admissions so the fact that you're asking about them is great.
I'm not an admissions officer so take everything I say with a huge grain of salt but I am a Y13 who applied to US unis - I didn't apply to any schools in the Ivy League, but I applied to several US of the same competitiveness, and have been blessed with some acceptances so I do know a bit.
Extracurriculars won't just be reviewed as boxes to tick off - they will be a key part in demonstrating your passions, ambition, past success etc - but, roughly speaking, with your ECs you're trying to show them four things:
- achievement
- leadership
- passion
- commitment/dedication
(Obviously, these four are all interlinked - it is virtually impossible to have achievement without dedication)
Achievement wise, you have some good stuff. Leadership definitely seems like your strongest area. Being the head of 3 clubs at your school and being in senior government at your school is awesome, but being head of that 1000 person organisation is very impressive! Passion is also evident as well. The only thing I can't comment on as I don't know how many hours a week and, how many weeks a year, and how many years (beginning with Year 10) you've participated in your ECs - these questions are part of the Common App, which is what you use to apply to most US universities.
As for whether your ECs are "good enough" is impossible to answer. Your ECs are definitely competitive and will certainly add to the rest of your application as your EC profile is really good, but the EC profiles of tons of other applicants are also really good. For schools like Harvard and Princeton, there are very few things a person can do that will pretty much "punch their ticket" into Yale (we're talking about being on Team GB or winning the international physics Olympiad*). This is not at all a reflection on you or your ECs, but of the simple fact that international acceptance rates for places like Harvard are likely around 1%, meaning that some extremely qualified applicants get rejected - it appears you know this though.
*I obviously don't mean that ONLY people who have that level of achievement in their ECs get into HYP, just that those are the only kinds of people who can get in on ECs alone (provided they have the grades to prove they can handle the work). For us mere mortals, haha, very strong ECs will need to be supplemented by other very strong areas of the application.
It look to me that you are "the MUN/econ person", so another factor that is out your hands is how many other "MUN/econ people" apply and what they look like. Colleges want a well-rounded class, so want to admit students with a huge variety of interests. I don't mean that you're necessarily directly competing with other "MUN/econ people", but more that what other applicants with similar passions ECs wise look like achievement, leadership, and dedication wise will probably influence how they view you. Dunno if that makes sense.
So yeah you probably didn't want all of that - I probably could have summed it up with: your ECs look very good, but so do those of tons of other applicants.
I'm normally much more articulate than this, haha. Anyways, hope some of that helps and let me know if you have any questions
Edit: One thing. According to their Common Data Sets, another thing US universities look at is volunteering. If you haven't, I highly highly recommend trying to get some volunteer work on your resume.