I consider class complexity to be most heavily influenced by how many subsystems your class includes and how many resources it gives you to track. I do not consider options as being nearly as large of a contributor.
Warlock (invocations, boon (2014)), sorcerer (sorcery points, metamagic), druid (wildshape), cleric (channel divinity), and bard (bardic inspiration) all have subsystems and/or resources to track in addition to their spells.
Wizard lacks subsystems and the closest they get to an additional resource to track is arcane recovery 1/day, and maybe signature spell at level 20.
Wizards only have to pick spells, and while my friend and I both shake our heads at wizard players who never change their spells from day to day, that part is optional rather than a resource that weighs into class power.
I know some people consider each spell to be a class feature based on more modern analysis (and maybe 4e affinity). In more old school play spells were more like weapons shared among classes (to some extent even pre-3e, when the sharing was more complicated).
IME even new and more casual players instinctively view spells more like weapons and have a harder time keeping track of additional class systems and resources than the list of spells on their character sheet. They might not be the best at remembering how each spell works, but they are even worse at remembering to use channel divinity or juggling sorcery points, etc.