on making changing spells outside of a long rest linked to difficulty if you did go that route I think it would need to be a more complex difficulty selection than there is in divinity. Like you'd need to be able to toggle different things instead of having a few discreet modes. Like with sliders that go [infinite spell slots -> spell slots automatically refill after combat -> spell slots refill on short rest -> spell slots only refill on long rest] and [change spells in combat -> change spells out of combat -> change spells on short rest -> change spells on long rest]. then you can mark certain options as recommended and you must be at least this difficulty for achievements or whatever. this would make for a much more customizable difficulty experience. you could also have an enemy/player HP multiplier or a shop cost or a vision cone or whatever slider.