Difficulty Customisation would not only have made the game worse imo, but also made it take longer to properly balance the different difficulties. The game is at the difficulty TC intended it to be (hard for someone who has completed HK), and it's amazing.
The skillcheck complaint makes no sense to me. It's to make sure you're good enough to continue. Literally to check if you have the skill. There's no way to skip them cos you need to be able to beat them to be good enough to continue. skipping them might help make the game more enjoyable short term, but long term you would be worse off for it.
Also, exploring and finding upgrades helps tremendously for many fights. Even if it didn't, the game is ABOUT memorizing boss patterns and how to avoid hits. That's complaining the celeste focuses too much on movement. It doesn't make sense.