I fixed it!
I thought X11 was pulling configs from /etc/X11/xorg.conf.d, but it was pulling them from /usr/share/X11/xorg.conf.d instead.
In this folder I created a file ‘99-fbdev.conf’ with the following inside:
Section "Device"
Identifier "myfb"
Driver "fbdev"
Option "fbdev" "/dev/fb1"
EndSection
Note: I’m still running on the image tainted by the LCD35-show script.
I found this solution here: https://www.raspberrypi.org/forums/viewtopic.php?t=66184