OK I'm stupid. I couldn't figure out why all my old cards were lingering even though I had followed these directions to a T.
After disabling Keychain on my iMac, iPad, iPhone, and Macbook, I kept re-enabling, and the same junk kept re-populating into the devices. Then I remembered I have a second iPhone 6 I use for my global phone. Repeated the steps but removed that one too this time, and when I turned it all back on, they were all blank.
Now when I add a card to 1 it adds the card to all devices, precisely what I was trying to accomplish.
Thanks everyone!