Is it worth dreaming abt cambridge

I don't see any reason not to, assuming you're doing A-level Maths and probably realistically FM.

I'm an international student in yr 12. My grades are avg now and I'm working on getting all A*s cus I'm capable of it and used to. I'm founder and Co lead of school's econ club (the club has only has 2 sessions so far due to holidays and circumstances so that doesn't affect anything ryt I won't have to show what I did there?) and working to start a newsletter dedicated to econ. I'm currently gonna focus on my finals in few months and gonna stack up on readings. So with A*s, good tmua score, good PS do I've a chance? Cus I see people with all 9s in hcse and insane work experiences and doubt themselves and idk if I'd stand against them. But I've also heard cambridge assess ur background so maybe that can help me ? BTW my gcses were mix of 7/8s and I didn't know it was that imp but I was sick so I did my best. School offers no fm

Yes, you have a good chance given you achieve all of those. However, it's much easier said than done. Many people have said the same as you before and have failed, so keep your head down and work hard.
Not having FM if not offered is theoretically fine, however do note that there is a plausible correlation between FM takers and eventual admits since FM takers are better positioned to do well in the TMUA compared to non-FM takers. If you are to do well, you should start practicing early.

I do maths but my school doesn't offer fm