Any system that does not manage that issue cannot be a meritocracy, because people who would be the best WILL be passed over for someone who had a luckier start.
What you can do is to offer a baseline equal chance, in the form of an accessible/affordable high quality public schooling system. It's one of the most sound investments any society can make.
Those that are able (capable, talented, willing) then have a fighting chance. Some will have to grind a little harder than others, but that's life. The point is that you get a chance to perform, and then you perform to your ability. You don't have to match the luckiest and most talented out there.
I myself was born in the lower working class, with no studying peers in my entire network, and dysfunctional parents. Yet, lucky to live in a country with a pretty decent public schooling system.
You know what shocked me most? How stupidly easy it is. The bar really is absurdly low. All I did was to consistently show up and pay attention in class. To take the education serious, which supposedly is a tall order nowadays.
Next, in my follow-up education I was advised to take the middle road, and to not aim too high. I dismissed the school director's advise and said that I will aim high. I'll simply cope by grinding through it. I'll study day and night if needed to match my superior peers.
Guess what? No such grinding was needed. Again I simply consistently showed up and paid attention, whilst my peers were drunk and couldn't bother to even show up. It was so stupendously easy that I could work so much on the side that I graduated debt-free.
If you don't do well on a test, do better. The world shouldn't give you an easier test because that's not the point of a test.
You know who scores well on test? Girls. Consistently better than boys. Why? They take the education more serious. They pay attention, ask questions, take notes, do their homework. Consistently and with discipline.
Sounds almost like...merit.
It's not about determinism at all, and pointing to people born in shitty situations who managed through luck and hard-work to overcome those difficulties does not mean everything is all-right, that is just silly.
In fact, especially for disadvantaged people merit is the dream maker.
But it doesn't, and this can be easily measured. Are you saying this is how you wished things were? Then I agree with you, that's what we should strive for. Are you saying this is how things are? That's just false (hilariously so).
Why must this be "combatted" at all?