How can I avoid accidental international roaming charges?

To prevent unintended data usage, label your physical SIM card (Domestic Plan) [Primary] and select [Secondary] for the “eSIM plan” at the time of installation.
Also, to block data usage based on a physical SIM card in advance, turn the [Cellular Data Switching] Off.

Apple

1. Go to [Settings] > [Cellular/Mobile Service]

2. Tap [Cellular Data/Mobile Data]

3. Toggle off [Allow Cellular Data Switching]

Samsung

1. Go to [Settings] > [Connections] > [SIM manager]
2. Toggle off [Data switching]

You can also contact your domestic carrier and ask about subscribing to a "Blocked International Roaming" service to prevent any accidental charges.