I'm wondering how easy the USPS is to route around.

In California's last election, there were 2,758 drop-box locations and 3,546 polling locations. In cities, these are often within walking distance. In rural areas, they might not be so convenient.

So it seems like, at least for returning ballots, any USPS snafus would affect rural voters (which skew Republican) more than urban?

Delivering is another question. I wonder if local governments could drop off ballots if they needed to?

No, the procedures are designed to reject more votes in blue areas, just like the procedures that somehow conclude that people named Williams, John, or Washington should be dropped from the rolls for being similarly named as other people named Williams, John, or Washington.