This seems a matter of framing. As drafted, this reads like a Guaranty Clause argument masquerading as a apportionment claim. The problem is that if you view this as a matter of how the state structured a single district election for a block of electors, the Reynolds v. Sims argument doesn’t work because every vote in a single district election has the same weight. That first past the post elections create a lot of losers isn’t necessarily a equal protection violation. Those electors could in theory take the minority’s concerns into account.
Things look really different if you frame this as a question of the state’s role in an intermediary for individual voting rights in what is actually a national election. For one, its not solely a question of the weight of my vote against my neighbor, but a question of the political agency my vote carries as compared to other voters nationally. This not only injures those voters whose votes aren’t “passed through” to the national election but it hurts the majority voters as well as the absence of competition deprives voters in states with firm majorities political agency.