Letting the system do things for the user is risky enough. Taking away the unambiguous way to turn the automatism on and off just adds insult to injury. This suggestion is not simplification, it is complication.

Personally I'm totally fine with light/dark/auto but if you really must simplify I see only two true ways:

1. Just offer light and dark. No auto. Whether auto works is out of your control anyway. Users who know and want auto have to switch manually.

2. Just offer auto on/off. Default is off and off is light mode. User who want dark mode will figure out how to configure their system.