the old T50 was (when it didn't produce a farm deck) the best way to compare my deck vs. other players' decks of comparable power. How do I do that now?
To me, this wreaks of a scheme to get players to grind, grind, grind instead of using their intelligence and skill. If you don't have nearly a full set of 6 of every card, you are most likely to get shafted by the T500 system. I have a very good deck that wins most of the time, but not when I get 5 random cards thrown into it.
I think you may have gotten something mixed up. You can use your own deck to play against the arena. The 5 cards from the oracle are only for when you want to create a deck for the AI to use in the arena.
Edit: Actually I'm now fairly confident that you got that mixed up. Otherwise you're making no sense. You said you're level 58, meaning that when T50 existed, you were likely never in the T50, since your score was probably not high enough. The new T500 is trying to allow anyone to be in the T500, and to do that everyone is allowed to build a deck for the AI, using the oracle card (so we won't just play against the same fire rush, ccyb, etc.). This deck that you build with the oracle card is
only for the AI to use. When you want to fight against someone in the arena, you still use whatever deck you usually use.