The 8086, which is based on the Z80, is not that much more complicated (even the opcode structure is similar, being an octal format), and same goes for the 80186. Things started becoming a bit more complex with the '286 and its protected mode, and then the '386+ with 32-bit and its various protected-mode enhancements really made it harder for a single person to master fully, but I think it was still mostly possible; the really hairy stuff started with 64-bit mode and all the extensions (e.g. virtualisation) that have been made to that since.

On the non-x86 side, modern ARM SoCs like the ones used in smartphones are not all that much simpler despite a less complex CPU core - they're still thousands of pages of documentation in total.