Most police services/forces have a ceiling on that. I worked in one where there was a combination. Not in the US, but the issues are similar and we had a union.
The union absolutely could not protect sworn officers who broke the law. And if a member was in legal trouble, the local branch voted on whether to provide legal support ( good lawyers paid by the union for as long as needed). I saw an officer charged with stealing (from a found wallet) where the branch voted not to provide that support.
My advice- pay police more and expect them to behave as professionals.
As a teenager in Thailand I saw police (1990) who didn't get paid enough to live- they relied on local community giving them rice, blankets, weapons.. or they ripped off folks -mostly from outside the community. Speeding tickets for foreigners etc.
I was paid ok to do my digital forensics- but doubled then tripled my wage in 2 years of leaving- doing ~ the same stuff (what people call incident response is built into your blood after a few years of policing).
I would also argue that I was much better as a police officer after 7 years than after 2, and much better at the specialization that I choose in the police- digital forensics- after 9 years, much better... and when i left @15 years I was much, much more valuable to the service than when I joined (skilled in forensics, understood the court process, could well react to emergencies, could write policy not just follow it etc)- but my pay had gone up maybe 30%.
But I didn't go up in rank because I didn't want to manage people, so my wage mostly capped.
And if anything- that is the issue with time based seniority- you force people to stop doing what they are good at and make them managers, with predictable results. (ie poor management)
Edited to not be a single wall of text