I suggest you consider DuckDuckGo, since it is incorporated in Safari natively. Its results are far more relevant that Google, whose sole reason for existing is to harvest your personal information at all costs, including the quality of its search product.
Like Qwant, it has very few advertisements. Unlike Qwant, it collects no data from its users.
Compare Qwant's treatment of personal information to DuckDuckGo's and draw your own conclusions:
Your data is stored on servers that belong solely to us and are never transmitted to external technical service providers. ...
However, judicial or administrative authorities may require the disclosure of data allowing the identification of a user.
When you search at DuckDuckGo, we don't know who you are and there is no way to tie your searches together.
When you access DuckDuckGo (or any Web site), your Web browser automatically sends information about your computer, e.g. your User agent and IP address.
Because this information could be used to link you to your searches, we do not log (store) it at all.
(emphases added)
Of course Google stores as much of your personal information as possible so they can sell it, and they make no attempt to hide that fact.
If you want to incorporate Qwant in Safari's "smart search field" the correct way to do that would be with a Safari Extension. Research Qwant to determine if they offer one. Beware that all Extensions modify Safari's behavior, and can result in unexpected operation.