So I guess due to hubris my latest system update ran the yay cache to the limit of my storage capacity and I couldn't reboot into a graphical environment. I suspect it was the electron update, BTW.
So do I need that rescue USB? Of course not. In systemd-boot, press e, end key, space and 1 and you're booted into a command line environment.
paccache -ruk0 nukes the pacman cache to be on the safe side
yay -Scc to clear the yay cache completely
And you're off to the races again.