Many good suggestions. However, I would like to offer a different approach.
You say you already own 600 Classical music recordings; half of which are opera. That’s a lot of music. Start out by listening to what you already have before spending a lot of your time and energy searching for and buying more recordings which you may or may not like……initially.
You also say that you want to “understand” as well as enjoy Classical music. To me, and not meaning to be presumptuous, this tells me something about you as a listener. There is a great deal to “understand” in Classical music, just as in any serious art form. This does not mean one has to get into any kind of heavy analysis. Understanding is sometimes the path to enjoyment.
Just as in Jazz, one of the most interesting aspects of Classical music which is worth having at least some understanding of is its history and evolution. From the Renaissance era to the Baroque and all the way to the present, the music composed in any one period builds on what came before. The music of the great composers in any one historical period was influenced by the music of composers who preceded them. Some understanding of this, if only by way of simple familiarity with the music, is particularly helpful for a newbie because music from the music’s earlier historical periods tends to be more accessible than that from later (more recent) periods. Some familiarity with the music from an earlier period helps with the appreciation of the music from a later period which tends to be more challenging to listen to.
I would start by reading the liner notes of recordings that you already have and do a somewhat casual cataloguing by the date of the music’s composition. Liner notes are a great source of info about the composer; historical and otherwise. IOW, as has been suggested, perhaps start with Bach, then Handel or Mozart before tackling Stravinsky or Alban Berg.
The world of Classical music is huge and is truly a wonder. Have fun and keep an open mind. Enjoy!
