It's really weird.
My game is running very well again, fingers crossed!
Like I said-- I deleted, or thought I deleted, Everything
But it still didn't work.
Then I deleted all my saves and saved sims,
then it ran perfectly!
So last night I brought back the save file I cared about, and put back in pretty much all of the CC I had before, and it still runs perfectly.
That said, I also updated overwatch and errortrap. I'd updated these immediately after sims into the future came out, but it looks like they have a newer version now.
It might be that the old save files or saved sims kept the CC from completely purging? I wish someone would explain how this makes sense (and perhaps how I can avoid future slowdowns).