More specifically, is taking up any job immoral according to Kant? Because by the Categorical Imperative, since we don't wish that everyone in the world became a philosopher/scientist/engineer etc.?
The answer in a nutshell is: don't be ridiculous.
You are taking the CA in far too crude a manner as a mandate of absolute "universality," almost as a synonym for behavioral cloning governing the specific "content" of judgments, as opposed to their "form." Kant was obviously a modern philosopher, for whom worldly contingency, radically specialized divisions of labor, and diversity of personality were givens.
The "hypothetical imperatives" of "if/then" judgments would apply to the content of most social interactions and decisions, with judgments referred to the more general CA as a "regulative ideal" or boundary condition. Similarly, we would never suppose that "equality under the law" requires everyone to have the same job.... or, say, to "not commit murder" in exactly the same way.
However, you do raise a good point. Off hand, not being a Kant expert, I cannot define exactly when judgments become "moral" judgments according to Kant. I assume others can answer. And the manner or extent to which some abstract concept of "equality" applies to individuals is always a crucial issue.
Kant takes "freedom" to be an inalienable aspect of human reason, moral status, and humanity in general. In the civil arena he favors, like most moderns, the maximum "freedom compatible with the freedom of others." And this civic "golden rule" would presumably constitute a moral indicator. However, Kant's Lutheran interpretation of "freedom" would not be that of the utilitarian or liberal.
But again, the CA is not some commandment to behave identically or to "act only as your neighbor acts." It is aimed at abstract moral contradictions, not the content and consequences of specific actions. It does not directly specify actions, but rather the constraints upon actions. It is, risking circularity, to act in such a way as to allow your neighbor to likewise act freely, rationally, and thus morally.