Update: I've managed to remove the thumbnails completely by using a custom catalog template file (following instructions in this thread
https://www.mobileread.com/forums/sh...d.php?t=118556). I just removed the text {thumb} from the default template and generated a new catalog with no thumbnails - I think I actually prefer it this way as the covers were always quite small and didn't look great on a black and white Kindle screen anyway.