How can I make the subscription widget show one-time purchase as the default option?
You can set the default to one-time purchase or subscription from the subscription widget app block settings. To do this follow these steps:
From your Shopify admin, navigate to Sales channels > Online Store > Themes.
Next to your theme name, click "Customize".
From the drop-down template menu at the top of the page, select the product template.
From the left-hand menu, click on the "Subscription widget" app block to access the settings.
Check or uncheck the box next to "Default to subscribe" depending on your preference.