It turns out that Apple's HomePod minis have had humidity and temperature sensors in them all along, Apple just hadn't activated them yet.
Once they did activate them, I found out that my upstairs was dry as a bone. Ever since my whole-home humidifier was disconnected (I was concerned about mold building up in my ducts), I've had regular plug-in humidifiers running. Downstairs. And apparently my thinking that the moisture was making its way upstairs was wrong.
So I bought a new humidifier for upstairs.
It's internet-connected like my main downstairs humidifier, which I named "Moisty Boi" (I got the idea from an internet meme). But now he's been renamed "Big Moisty Boi" and the new one is "Lil' Moisty Boi"...
Isn't it amazing how absolutely everything is internet-connected now? My new water heater (along with the rest of my home)... and now my Moisty Bois.
All that's left is my toilet, I guess.