Because you're still hormone sensitive, I think starting Zytiga now is a great idea. If you're already taking Xgeva to maintain bone density while on ADT, you should add Celebrex to it. I don't think there's any justification for the idea of saving stuff for later - the survival increases are generally much larger when used earlier. Later, there will be other medicines.
You might also consider the clinical trials described here:
/pcnrv.blogspot.com/2017/06/newly-diagnosed-metastatic-m1-but-still.html