Hmm yeah this seems like a fundamental misunderstanding of spellcasters on their part. In general they have two objectives: stay alive, and cast all their spells in a productive way. They should try and end every adventure day with no spells left. In my experience there is little value in hoarding spells, and if they want to do that suggest that they buy scrolls of the spells they always want on hand. Spell slots aren’t much of a limited resource assuming they can rest when they want or near enough, with scrolls filling the gap.
That said, it sounds like casters just aren’t for them. Have them play a fighter, seriously. They will love it I promise.
I just got Sleeping Gods and it’s fantastic so far, a really wild game concept.
We’ve also been playing Dune imperium for a while now.