I 100% agree this process is DREADFUL (shame on apple) but I have found the solution (though it only works on real device).
Step 1: create a new sandbox tester (with random fake email address and BS 'secure' password).
Step 2: open Settings in your device, then navigate to AppStore
Step 3: scroll down to Sandbox Account Sign In with your newly created account (you may have to first sign out of current account).
*IMPORTANT
You may be prompted for two-factor auth. If this occurs, tap the 'Other Options', then select 'Don't Upgrade'. This will skip two-step auth and sign you in.
Step 4: Open your app on the device you did all this and make the purchase.