There isn’t any way to directly override the field type used for the default value in ACF, at least not that I can find.
You could add a new fields setting using https://www.advancedcustomfields.com/resources/adding-custom-settings-fields/. But I don’t know that the WYSIWYG field will actually function on the ACF field group page. You would probably need to load additional scripts in order to get it to work similar to the way extra scripts need to be added to get the image field to work described here https://acfextras.com/default-image-for-image-field/
If you can add the WYSIWYG field setting and if you can get it to work, you could then use an acf/prepare_field filter https://www.advancedcustomfields.com/resources/acfprepare_field/ to pull the value from the new setting field and put it into the standard ACF default_value for the field.
