Thanks for the feedback!
Yeah, deeper runs will definitely wear down your tools more, and that means more resources spent on repairs. So sometimes we'll have to perform "resource runs" meant to stock up on resources, or goal-oriented runs aimed at progressing through game objectives or unlocking achievements.
Diamond chest spawns have increased, and diamonds themselves are a bit more common now, especially in the magma zone.
For the Enlightenment dojo training, each level adds +5 to your base light radius, for a total of +30 at max level. It's meant to be more of a late-game quality-of-life boost rather than an extremely impactful upgrade. Giving too much light would throw off the balance and make the darkness mechanic feel trivial, so it’s tuned to be helpful without removing the challenge entirely.
And congrats on beating the level with Fern! (and not even using Flask at that)