If someone mails in my absentee ballot and I don’t complain, how do you detect that voter fraud?

Or if someone knows their friend is sick and votes without an id, how do you detect that?

It seems like there are currently many ways to vote illegally that don’t get detected.

We already handle all of that, comrade. Every corner case floating around your brain was floating around someone else's brain a long time ago. Most of this is covered in high school in the US, and it's all enforced by volunteers from across the political spectrum.

Our documented examples of voter fraud come from a time when in-person voting was the only option, again something we teach in school, while the modern concerns from security professionals focus almost entirely on electronic voting machines.

Perhaps so, but you still have to show that it is happening, not merely that it is possible. Moreover, you have to show that whatever cures you propose are both 1/proportional to the harm and 2/minimize undesirable side effects. (One challenge with the latter is that for some people, those side effects are actually desirable.)

Can you describe specifically how someone finds enough complaint-free absentee ballots and sick friends to vote at any meaningful scale?

Doing this even 10 times seems unbelievably hard.