My download seems to be throttling itself because it’s topping out my removable disc usage… But this game is on the SSD

Do you have a steam library on the slow drive? Apparently steam usually puts in-progress downloads to the same drive as the game destination but if there's not enough room or maybe just at random steam will download to the wrong drive.
This is a few years old now but there's some discussion here https://steamcommunity.com/discussions/forum/1/1646544161774522444/