I live in California, where this is a problem. Part of the issue is "catch and release", which was a policy change in CA a few years back. The problem they were trying to solve was the incarceration of innocent people, who lost their job due to the incarceration. There were lots of negative side effects to this policy change.

The news is full of anecdotes about those being released committing much worse crimes. The problem you are going to have is almost all of these are about citizens, not immigrants.

I suspect most in sanctuary cities believe that immigrants should be treated about the same as citizens. If you have a crime problem with immigrants, you likely have the same, but bigger, problem with citizens. The solution is to fix the underlying crime problem, not pick on one particular group (be in racial, ethnic, or immigrant).