I'm an offer holder for entry this october (STEP 3 on monday pray for me haha) but I think the important question isn't whether or not you're good enough but whether or not you think the cambridge is the right place for you to study maths. It's a very intensive course and they expect a lot from you in the course and term, but if you really do enjoy maths, and you think that you'd fit in there, then I think it's worth at least applying because if you think that you realistically are prepared for that, then you will be able to put in the work now definitely. STEP is a very daunting task but it is definitely doable, my school offers me basically no support and I wasn't given an opportunity to enter any Olympiads or maths challenges, so you definitely don't need to worry about having it as experience (although it definitely would help), but I don't think I'm particularly amazing at maths compared to some of the other offer holders that I've spoken too but I've been doing STEP non stop for a while now and it has definitely helped me a lot, and because I've done so much extra work preparing for STEP, my maths in general is definitely way above where it would be had I not, and just that in itself is an amazing thing to take from it.
that was really long so summarised;
if you think the course is right for you and you'd fit in then apply and you'll be able to work hard enough to get in,
even if you apply and don't get in it is an amazing opportunity with interviews and preparing in general
anyone can get in no matter the background if they work hard enough imo