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.

It seems less hard if you datamine the shit out of everything, exfiltrate the social security database, and feed it into a computer. Get the historical voting records. SELECT address FROM voters that haven't voted in 10 years. Send someone to follow the mailman and steal ballots from that address. Or simply don't mail them out in the first place. They're not likely to notice to complain in the first place.

Not that I think the election was rigged, but if you think it's "unbelievably hard", I think that's a failure of imagination.