The Beginner’s Master List contains all useful links – not only for beginners but also for those who are already acquainted with CS itself (I found myself still looking for the thread multiple times).
And personal opinion, but your plan for hardcore mode seems… too hard. Even ignoring the potential timed-decision mechanic, the restriction is too high; you limit the amount of skill obtained and increase the skill requirement. Keep in mind that number-based balance issue is exponential and is not simple addition.
Balancing is not about setting up arbitrary numbers. You need to consider your difficulty curve, mechanic and narrative progression, and setting up interesting end-state (winning or failing must be satisfying, unless you know what you want to do).
And in case someone brings up Dark Souls
From Software understands their audience and their product, so they intentionally make their game with a mountain Everest as its difficulty curve. They know what they’re doing.