That doesn't sound very difficult to me.
I agree with you that no one person can make a big difference. Whatever affect this has is cumulative of all the separate individuals who do it. And by the nature of this, it's easy to do and not get caught, so we can't know how many that is.
for it to have any meaningful effect, you need at least 10,000 people doing it in one state just to get about 10,000 votes.
sorry, but things like that don't happen without somebody getting caught. it requires organization, discipline, and the victims not noticing.
because there's only 2 ways you can steal a ballot
-the victim doesn't want to vote and doesn't care (or dead)
-the victim does want to vote, so there's a discrepancy (you'll get caught, or at least there would be a record of attempted fraud)
with such high turn out, you have to also consider safe states vs swing states. in safe states, it doesn't matter, you'll never get enough fake votes. in swing states, people vote and notice, this dynamic is pretty double edged, and statistics with some auditing can easily reveal any meaningful differences.
as an example : if too many ballots are "stolen" then you'd expect the victims to speak out. But so far, it seems it's just people complaining there's fraud but nobody who had their vote stolen has come forward.