A violation of a departmental policy alone does not provide grounds for overcoming qualified immunity.
Qualified immunity is a defense to a Section 1983 civil rights lawsuit. The prima facie case for a 1983 lawsuit requires showing that someone, under color of law, intentionally violated a constitutional right.
Qualified immunity further requires that the constitutional right be "well-established". A constitutional right is "well-established" if a court case which is a binding precedent in the place where the incident occurred has held that a constitutional right was violated in circumstances sufficiently factually similar to the case that forms the basis of the lawsuit that a reasonable person who was aware of that case would understand that their actions violated a constitutional right.
There is no constitutional right to have law enforcement follow its own internal policies. If the policy also articulates a well-established constitutional right (as many law enforcement policies do) then it could state a claim for relief. But if the policy goes beyond what the constitution requires, then you don't even reach the question of qualified immunity because the claim fails for lack of a prima facie case of a Section 1983 claim.
It is possible that there could be a private cause of action under state or local law for violating a departmental policy, but that would be unusual. It would be far more common for state and local law claims other than a 1983 claim to be barred by state governmental immunity doctrines.
Usually, the main legal consequence of a law enforcement officer violating a departmental policy that does not coincide with a well-established constitutional right is that the law enforcement agency that employs the law enforcement officer will have good cause to discipline or fire the officer for violating the policy. Whether the officer may be fired or only disciplined for violating the policy largely depends upon the requirements of the civil service rules applicable to that officer and/or the relevant collective bargaining agreement if the office is a member of a police officer's union.