Switching from Steam Big Picture to KDE Plasma desktop?

One of my gaming rigs is in my living room, connected to a receiver and TV. I'd like to set it up to auto-login to Steam Big Picture mode (I do have this working, following the SDDM instructions), the problem is if I want to exit Big Picture and log in to Plasma. Steam's "Switch to Desktop" option doesn't work, it just hangs until I cancel it, but I figure there must be a series of commands or a script I could write that would do the same thing.

I haven't been able to find a solution for this as its fairly bespoke. Using TTY, can anybody tell me how I could exit Big Picture mode, returning to SDDM where I could log in to Plasma? Or maybe even just launch a Plasma session from TTY and maybe kill the Big Picture session?