Process finished, device successfully rebooted by itself. I'm now on 9.2.0-20U, no noticeable issues at the moment.
Here's the info;
Model: 3DS XL, Black
Firmware: Started at 10.3.0U, downgraded to 9.2.0-20U successfully, by the looks of it.
Results: Downgrade successful, it would seem. Took me a few tries to get into the installer (around 10), but I managed to do it by exiting ftpBrony NORMALLY (as in pressing B, not force closing with L+R+Down+B). Installation finished in a few minutes and the device rebooted itself after ten seconds. No issues even though I used Shufflehax as my entrypoint an Browserhax as my initial entrypoint. You can refer back to my original post in this thread to see the exact steps I took to make sure this would go as safe as possible if you think it'll help others.
I can provide screenshots if you need proof, although I don't have a screenshot of me being on 10.3.0, sorry.