"Core D&D Rules" doesn't make sense as a difficulty level. It's about having a coherent system of rules. The current state of the game is taking two different systems and mashing them together in a way that makes the whole game suffer. Asking for rules that make sense as a single system isn't about setting the game difficulty, it's about setting player expectations, creating verisimilitude, and making the game fun. Once you have that, then there should be a variety of difficulty levels.
I want the rules to make sense and work together. That has no bearing on how difficult I want the game to be.
