I don't think culture problems across a population as large as the collective police forces of our country will change from the one-off murder trial, hand slap, or sensitivity training. The culture needs to be addressed daily across the population from management at all levels. I am not sure how to do it, but it seems that somethings are simple, like daily management to enforce the end of the Thin Blue Line mantra. Lines divide people. Police can't be allowed to feel justified in feeling separated from those they police.
Other things like enforcement of operational body cameras 100% of the time. If the departments managed their employees to treat their cam like they do their gun, in ensuring it is in good operation before going on duty, the bullshit about body cameras not being on during incidents will go away.
I think a big part of the problem is with a union that is too strong and has too much control, influence, and money. Low salaries drawing poor candidates and a strong union making it hard to cull out the bad ones early enough is a huge issue. They also need a crap load more training. Both upfront at their academy, and continuing ed training on the job.
A lot of the issues could most likely be helped with a lot more money that is managed by good managers who have the ability to fire poor officers. Unfortunately, we find ourselves in a world where de-escalation is something a pussy does and where problems are thought to solve themselves once to take tax money away, so nothing is going to change.
