Make sure your video driver is up to date.
But drivers can't be updated manually on Mac osx, it's done through the OS update and I have the lastest update (The issue was present even on the previous Mac os version)
I am sorry for the inconvenience caused to you, I see that you have tried most of the steps.
Try updating Lightroom to the most recent version: Lightroom 6.6.
In case it does not help, I would request you to enable Root Account once, log out from your account and log in to Root Account and see if the application still crashes or not.
How to enable root account in MAC: Enabling and using the "root" user in OS X - Apple Support
Thanks a lot Tanuj, As I learned the hard way not to jump on updates as soon as they are launched I will wait a bit for the 6.6 to see the feedbacks of people and anyway I don't believe it's an update thing as I tried 3 or 4 different Lightroom 6 updates and nothing happened.
But meanwhile I tried that root access thing and seems like Lightroom is not crashing for now it takes a while sometime to crash after clearing the cache or resetting so I'm not getting excited, I will update you in case it crashes.
Update: Went back to crashing after 2 days working fine