I think one of the most important, and difficult, jobs of parenting is knowing whether you have the kind of child who will thrive on being pushed to excell or not. Being born by a certain set of parents in a certain family does not automatically equate to a certain type of brain.
My eldest brother was made quite unhappy by my parents' expectations until they realised that he was not them and never would be. Otoh my younger brother and myself were very like their dreams and benefited from having those expectations. My eldest brother only really began to feel good about himself when he went down a totally unexpected, non-academic path and found something he could really do. He now earns a comfortable salary doing a responsible job that he knows he is good at.
"I want my DCs to go to a good university and from there, to get a good job. That isn't going to happen if they aim to be at the national average."
You are not allowing for the fact that it is what your dcs want that matters.
As a parent you have only got control for so long and if you try to push your luck beyond that, you can damage your relationship for a long time.
My FIL was very upset when dh decided to become an archaeologist (digging, not academic) and you can see his pov: digging in the mud is hardly the route to wealth and security. But dh has been very happy, he has been lucky enough to have relative job security and at the end of the day it was his life and he was not going to back down. Sadly, it took years for both of them to get over harsh things that had been said on either side.
I would also like to add, as a university teacher, that there is nothing more disheartening than trying to get a spark of independent thinking out of a student who has no passion of their own for the subject but is only there because mummy and daddy wanted them to have a degree at a good university. The only sparks you tend to get from that kind of student is the anger when they realise they've got a Third.
Otoh students who are there because they wanted to be there, students who make up their own minds, students who do not let other people be in charge of their future - those students can go far.
I try to ensure that my children are constantly exposed to good learning experience, I try to show them that learning is valued and considered fun by the adults around them. I encourage them to work hard at school because doing a job (any job) well is a virtue in its own right. When required I do my best to advise them on study and career opportunities. But I do not for a moment imagine that it will be my job to tell them what to do with their lives.