This was already suggested.
I want to emphasize how qwant is growing and has a philosophy close to opera's.
Of course, we can add qwant as an "other search engine". But to use it we then need to tape "q" before every search or use the extension. And it does not work with opera's search function (when text is highlighted).
This would be a nice feature for opera and I don't think it is very complicated to implement.