I tried uninstalling drivers, system restore, Windows 10’s Network Reset, I even plugged a different adapter, but for whatever reason the download speed was still seemingly capped at this seemingly arbitrary value.

The only thing that finally worked was I downloaded this program, and optimized the adapter.

https://www.speedguide.net/downloads.php

Clicked on Optimal, then Apply Changes, restarted, and the problems were gone.

So yeah, if this ever happens to you, try this before pulling your hairs out 🙌