You’re right that trying to create a third US party in the way that you’d do it in a representative parliamentary system (build support slowly, try to gain a couple of seats and look for balance-of-power/coalition opportunities) ain’t gonna work.
That’s not the only way to do it, though.
Historically, US parties have been annihilated and replaced when the spineless careerists betray their base once too often (Whigs -> Republicans). But it happens in a rapid revolutionary surge, not as a slowly-building movement of “realistic” business-as-usual politics.
If a third party gets up to the point that it’s a major force, it won’t be splitting the vote with the Democrats, because the Democratic Party will be dead and gone by that point. One party or the other would be annihilated after a single election cycle.
