How can I force my iMac to use iMessage NOT SMS
Couple of things to try:


On your iPhone, choose Settings > Messages > Send as SMS and toggle the setting to Off.
When sending an iMessage to your wife, try using her Apple ID instead of her phone number. You can check what numbers or email addresses she can use in Settings > Messages > Send &amp; Receive.
Check your wife's mobile data connectivity/data roaming. She won't be able to access iMessage if she has no internet. See Settings > Mobile > Mobile Data and make sure the setting is On (it may be Cellular in the US if she has changed locale).
Probably obvious, but make sure your wife has iMessage enabled. Check Settings > Messages > iMessage and make sure the setting is On.


Note with the second point there. There used to be an issue whereby Messages on macOS would group the chats separately if you've sent messages to different addresses of the same contact. Not sure if this has been resolved, but it was frustrating.