Do you think you'll find an answer, rationally speaking (that is, using the tools of reason and logic)? Notably, Godel's incompleteness theorems suggest these tools may be insufficient. More specifically, if you try to rationally "justify" what you ought to be doing, you will always be able to - legitimately! - impugn its logical foundation.
This suggests that consciously reasoning about what someone's fundamental purpose is may not be the best way to go about things. Rather, just as how we make most decisions, an intuitive and heuristical approach may be better. In other words, "do what feels right."
What feels right may materialize as a family, as charity, as building a business, or even building a career. My point is that all of these can be challenged as "deferring existential questions" - again legitimately - because they cannot be justified rationally. (All of these, equally, can be criticized in hindsight as being "wrong": wrong marriage decision, wrong way to raise children, wrong work-life balance, wrong charitable/activist cause.)
As a result, I don't think the criticism that a career - even an all-consuming one (as a family may be) - is somehow less justified. This is not to invite pure relativism: some things do seem to be more meaningful than others.
But! It is precisely that this "seems to be" is intuitive, and not rational, which constitutes the foundation of our belief. And so if a career-focus seems "intuitive", then I think it's meaningful enough.
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EDIT: One other thing to note is that people who are fairly content with their lives don't often ponder _why_ their life is meaningful. They've specified a priori assumptions for what makes it meaningful (e.g. charitable giving, helping others), or intuitively feel what is meaningful (e.g. family, friends, happiness), and that is that. It's precisely the people who end up endlessly questioning these assumptions - facing the meaning of their life head on - that seem to be most depressed. This is because it's an intellectual deadend - it cannot be deduced rationally. One way, then, to answer this existential question is to simply defer: don't question, do what feels right, and avoid what doesn't.