Thanks for the your update!
I found that I rejected the possibility of something in my home directory being the cause of my issues too early (though not without reason, but that doesn’t matter here). Turns out, a new account I created loads a perfectly functioning plasmashell.
I then symlinked my old .config directory into the new account’s homedir, and boom: it’s broken again. I’m currently in the process of narrowing down the issue to a specific file or directory and will update you accordingly.