Usually an SI is written vaguely enough that even if it is the author you can’t tell. Once you’re past the first chapter the fact that they’re an SI rarely matters to the story and it’s just a Mary/Gary Stu OC to act as the reader’s avatar.

Meta knowledge usually acts as the motivation to accumulate power and be prepared for inevitable war etc but is rarely deployed in the sense of changing the timeline. Usually the SI tries to stick to the stations of canon and not interfere until they have accumulated enough power to be untouchable, as changing events results in butterfly effect that makes the world unpredictable.

 

When I read fiction, the protagonist is someone I empathize with, root for, feel bad for and so on, but I don’t usually consider them an avatar for myself. My favorite protagonists are more like people I’d want to be friends with, and reading is like hearing about this crazy thing that happened to them.