> I'm not sure that unequal relationship between employer and employee is a feature of capitalism per se
It is.
It is also a feature, to a greater or lesser extent, of many systems that are downstream descendants of capitalism and haven’t replaced the elements that produce that feature.
It is also a feature of the upstream immediate predecessor of capitalism (feudalism, in which control of the means of production is unified with land ownership and typically executive, and often judicial and legislative, power over those on it) in an even starker form.
> even in a market socialist system
“even”? Market socialism is a very close relative and (conceptually) descendant of capitalism, not some distant and opposed system.
> in which labor receives all that it produces, any individual worker depends on their employer more heavily than their employer depends on them, so you still have an imbalance of power.
In market socialism, the employer is an agent of the employees, who (in many versions) each individually hold guaranteed rights with respect to it much the same as citizens of a liberal democratic regime do against the State. Both the universal elimination of the employer fundamentally serving an interest outside of its employees and also the common (but not universal, “market socialism” is a broad category with lots of variations) guarantees of individual rights to employees exist specifically to mitigate the adverse effects of the employer/employee power balance that it retains from capitalism.