Nobody who would have otherwise voted for Trump is going to be convinced to vote for Stein. Every vote she gets IS one that was much more aligned with Harris.
This isn't a question of ALL third parties, but there aren't any right-aligned third parties making any kind of a meaningful run.
In Michigan Gary Johnson got 172,136 votes, in Pennsylvania he got 146,715, and in Wisconsin he got 106,674. If all Greens voted Clinton and all Libertarians voted Trump then New Mexico would've only been won by Clinton with around 1,000 votes, Colorado would've also been nearly Trump. Nevada, New Hampshire, and Minnesota would've been won by Trump. Maine might've gone majority Trump.
Third parties hurt Trump more than they help him, because Libertarians would not have voted Clinton.
If all third-party candidates had to vote for one of the two main candidates, I think nearly all of the Green Party votes would go to the Democrat, while the Libertarian votes would be much more of a split.
while the Libertarian votes would be much more of a split.
Maybe an 80/20 split at best, but the GOP has always been the more libertarian aligned party, going back to Barry Goldwater, Ron Paul, Rand Paul, and now Thomas Massie
The Green party gets more attention in left-leaning circles because there are people sympathetic to it and there are people who want to blame them for the Democrats losing. It's not because they're actually more popular than the Libertarian party, which regularly gets like 3 times as many votes.
So it's completely wrong to say that "there aren't any right-wing third parties making any kind of a meaningful run." It's just that your perception of how popular the Libertarian party is compared to the Greens is distorted.
How? By the same math as OP, the Libertarian Party splitting the vote cost Trump Arizona, Georgia, and Wisconsin in 2020. That's 37 EC votes which would've been enough to make the election an exact tie.