> There should be a national registry of officers fired for misconduct that makes them ineligible for rehire in all jurisdictions

There is/was (though Trump has largely de-fanged it) - tracks officers fired or who resigned in lieu of termination.

> but police unions and the politicians they back would never let it happen.

Correct again. The Unions that represent ~70% of police across the country have managed to get into their CBAs that the database is not to be used for hiring or promotion decisions (i.e. not to be used).