is there a way to toggle my phone to default to sending as a text
until she has roaming availability?
No. This is not possible.
There is no way for your iPhone, Mac, or even Apple's iMessage servers to know whether your wife's phone is roaming or not.
Roaming is a carrier function designed to allow the user to seamlessly use their home carrier services on a different carriers network. The phone doesn't notify any other phones it's not on its home network.
roaming refers to the ability for a cellular customer to automatically make and receive voice calls, send and receive data, or access other services, including home data services, when traveling outside the geographical coverage area of the home network, by means of using a visited network.
If roaming is an issue, you should use a 3rd party app like WhatsApp or have your wife turn off roaming altogether.