Li Shenron
Legend
I don't really see at first blush where they could simplify further without breaking stuff.
One possibility (for character complexity) would be to rethink whether one class' own special features really represent different things, and if not then merge them together.
For example, Cleric's spells and energy channeling are conceptually very similar, they both represent magical/supernatural powers granted by her deity. Furthermore, they both work on a daily basis, they both scale by character level, they both depend on the chosen faith... so why couldn't they be the same class feature? That would mean one thing to track/manage instead of two.
But for in-game mechanics, I think the game is almost as simple as it can be... at least combat. For a beginner's group, the DM and player can simply ignore every combat action besides the 4-5 most basic ones, but the combat structure and action economy really is very simple already. And for tasks/skills, the resolution mechanic itself is supersimple and very "narrative" in nature.