I'll post again in a bit with some tracert results, but for now:

I tried Janyd's recommendation, and it feels like it helped a little but I'm also very much still experiencing XIV-specific lag that's making the game unplayable.
Shout out to the Frontlines team that had to deal with me while I learned the Win10 repair wasn't my solution. (We came in 2nd, and my end score was an unprecedented 0-4-0; I've literally never failed to rack up even a single Assist, this lag robbed me of multiple aoe hit credits lol)