Currently, the only way to do that in the default setup for Safari is to use the Web tab after the Google search. There are other Extensions available for Safari to create custom search engines, but I have not tried them. One example is here and does have good reviews. I don't know anything else about this extension, other than a simple search in the App Store, so sorry I am not able to provide a recommendation:
https://apps.apple.com/in/app/custom-search-engine/id1588019370
Google is pushing AI hard and has taken a different approach where it is usually an opt-in feature and instead is putting it out in default searches.