Thanks so much for the tip @LoreNG, that completely solved my problem! I chopped all of my USB cables and twisted them together with some breadboard jumper wires. Plugged them in to the VIN and GND pins:
This completely fixed the WiFi signal, so I’m getting -63 dBm instead of -89 dBm in my ensuite bathroom:
I won’t buy any more of these boards, but I’m really glad I was able to salvage them and use them as ESPresense base stations. Hopefully some of the other ones I ordered will work fine when they’re powered via micro USB.