It might be possible to boot into a bootable image from grub so you don't need to set up another bootable partition.
Or you could disable your display manager in systemd. This will start in console, then if you want X just run startx.
Or you could change your display manager to Lemur, which supports X, Wayland, and TTY sessions.
Or you could just press control-alt-F2 at the login screen to switch to a console.
I don't have any need to edit markdown, but I sometimes use Marker: “Simple yet robust Markdown editor made with GTK” as a viewer.