I can finally get the app to run on the simulator! (I’ll get to real devices next) I was able to do it with Carthage installed at /usr/local/bin/carthage - I did not have to move it to the WebDriverAgent folder.
First I reinstalled appium-xcuitest-driver:
npm install appium-xcuitest-driver
I noticed that the app was trying to open was wasn’t quite getting there. Turns out there was a miscommunication over the bundle ID. I updated that and now I can run the app on the simulator. So definitely double check bundle id, and make sure you’re pointing to .app / .ipa as appropriate. (Just adding this because it’s a common mistake for me.)