One d20 game that also leaps to my mind from this description would also be Index Card RPG (Master Edition).But yeah, my players do love the safety of 5e (they still die a lot in 5e). They love the idea, the fantasy, behind a moon druid, a rune knight, an astral monk and a clockwork sorcerer, but honestly, they dont understand how they work. All those rules and options are way too much for them; they barely remember which die to roll in-between games
So that's why I'm looking for a OSR-ish game that offers many cool character fantasies, without overloading the character sheet. Or a game that's easy enough to modify to create those concepts without spending hours on it. Hence, Shadowdark.
On the other end of the spectrum, I could also see Shadow of the Demon Lord/Weird Wizard also working, but I'm not sure if that's too radical of a departure for your group or if it's too much ability over-hang. It at least does a lot of it in smaller chunks and doses.