Rav Moshe, in teshuvos, when referring to Reform or Conservative Rabbis would spell out the word “rabbi” as if it was an english word–something like “רעבבי”
I view “Rabbi” as a title we use when interacting with the secular world. It should be used to denote some position of religious authority, whether in a shul, yeshivah, kashrus org., etc.
Internally, I prefer to use Rav and I use that for anyone that I would feel comfortable relying on for a halachic decision.