Thank you. I used an 1.8.2 I found on that forum (BOOX C67ML V1.8.2 is the name of the download link). I don't know if it's the same as the one you sent me, which was the same link as the one I've tried using at first. Notice that you sent me the same link as the one I sent you at first. The one at Google Drive.
I forgot to mention

that Android recovery mode also failed updating directly to that 1.8.2. When I already had 1.8.1 installed, I tried updating to 1.8.2 with the new download I found, using the update option on the Boox. It worked! Cloud update didn't work
The Boox is way better with this new firmware.