Open fullscreen apps to left of Desktop 1 space In Settings -> Mission Control, you have the option to open a new space to the far right or to the right of the current space via the "Automatically rearrange Spaces based on most recent use" checkbox.

What is a way, perhaps through a terminal command or otherwise, to have new spaces open to the left of Desktop 1 (primary space)?
Up to the current macOS (10.14.2), there is no way to change the default position of full-screen application.