A bit too late, I always wanted to do it.
LSPD First Response (LSPDFR) is the police modification for the PC version of Grand Theft Auto V which completely transforms the game into a law enforcement sim.
It’s a bit challenging to set up but no more so than mods for other games. I don’t know how much replayability you’ll really add but it’s fun for a while depending on the extra mods you add.