@kjemikael I feel exactly the same way. I've added it as a search engine, which is easy enough in Opera, but you Cannot make it the default which is just stupid. Looking around, I can see setting custom search engines as default has been requested from as far back as 2015!
I love opera dearly, but until this is resolved and either Brave added as a search engine or the ability to set custom SE's as default is resolved, I too may have to switch to the Brave browser. Its not something I want to do, but this is a pretty mission critical function.