So to me, I wanna clarify they're interrelated, you do need more complexity to have more depth, generally-- but the amount of depth you buy with any amount of complexity is down to the 'elegance' of the design, 'efficiency' might be the right word. Pathfinder 1e (3.5 really) was very deep because it had a mountain of complexity, but none of the complexity was that efficient. Whereas I view Pathfinder 2e as having a similar depth, but far less complexity, but it still needs some to do what it does. For point of reference, I view 5e as having very little depth per complexity, but it also has very little complexity. How much complexity is justifiable, even if its efficient, is of course down to taste and how much you value the kind of depth you can get from a rules system (which isn't the be all / end all.)
My biggest problem with 5e, now that I think about it, is that its so much more complex than say PBTA or other rules lite systems... but its not especially deeper than them, while managing to be a little simpler than other d20 games of its ilk, but losing out massively in the depth that arguably makes them attractive in the first place.