Re: NVDA and speaking Capital letter
Although I can't speak definitively, I'd have to guess that what determines whether an acronym in all capital letters gets spoken as a word is whether the letters in the sequence can be pronounced under the rules of English phonotactics or not. Using your example D A X together can be spoken as dacks. There is no way, under the rules of English phonotactics that SPX can be pronounced as a word. It has to be rendered letter by letter. Another example is NASA, which can be pronounced as a word, versus LPG, which cannot.
I know of regular expressions you could use to recognize all capital letter acronyms in a fixed number of characters, say 3 or 4 or 5, and pronounce those as their individual letters, but I do not know how to do this for an indeterminate number of sequential capital letters. You'd also have to be somewhat careful about this, since it can quickly drive you crazy if you're reading a document that has lots of pronouncable acronyms and you end up having them broken into letters every time. It would drive me insane to hear NASA pronounced as N A S A were I reading anything that refers to the U.S. National Aeronautical and Space Administration repeatedly by its well-known acronym.
Brian - Windows 10 Pro, 64-Bit, Version 1909, Build 18363
Science has become just another voice in the room; it has lost its platform. Now, you simply declare your own truth.
~ Dr. Paul A. Offit, in New York Times article, How Anti-Vaccine Sentiment Took Hold in the United States, September 23, 2019