For anyone looking for a GUI for this workflow, I made SimParcel, a free, MIT-licensed macOS app. Drop files onto it, select a simulator, and click “Send to Simulator.” Photos and videos are imported into Photos using simctl addmedia. Other file types, including GPX, CSV and XML, are copied into the Files app’s local storage and appear under Files → On My iPhone, where you can select them from your app’s document picker. I’m the author and built it after running into the same issue: https://github.com/VadimKat/SimParcel

I ran into the same problem and made SimParcel, a free, open-source macOS app that provides a GUI for this workflow. Drop your data file onto it, select the simulator, and click “Send to Simulator.” It copies ordinary files into the Files app’s local storage, where they appear under Files → On My iPhone. That should be useful for the test-data backup you described. It uses the same Files app group container approach as your workaround and also supports importing photos and videos through simctl. I’m the author: https://github.com/VadimKat/SimParcel