Setup full disk encryption using LVM on LUKS (single drive) with remote SSH for an already setup/running Raspbian stretch system. Using this guide there is no need to connect a screen or keyboard to the rpi at any stage since we will setup remote SSH at the same time (at present no other guides available tell you that this is possible).
To be safe, backup SD card (using another computer is easiest in my case):
sudo dd bs=4M if=/dev/<rpi_sdcard> | gzip > rpibackup.img.gz
or use my fish shell function.