I ended up using the directions from your first link, the raspberrypi.org, with a slight modification. When I tried following the directions exactly it did not work. Then I noticed the fstab file uses PARTUUID not UUID... at least on all the Pi3 models that I have. So I changed that from UUID to PARTUUID and it worked!
I use EXT4 as well. Basically I setup my Pi, install the updates, install GDEBI and GPARTED, partition the external drive to EXT4 with GPARTED, edit FSTAB for auto mount, install Core, and point the /ochives directory to my external drive.