I used Qwant years ago, but don't remember why I stopped. Maybe they introduced ads like Google, or something more annoying. But it's not 100% european, they use Google and Bing engines (just like most private search engines) to show you results. Also, there are some articles talking about how sketchy it really is (French government public funding, links with Chinese companies, and more).
About Vivaldi, it depends on your devices. In my experience, Vivaldi for Android works fine on weak hardware, faster than Brave and Firefox, but slower than Chrome. Using it on PC always was a headache, it's the worst optimized browser I've used. They care more about productivity features than performance or privacy. It can feel bloated.
I understand your concerns about headquarters and privacy. I think the strongest european options for browsing and searching are Vivaldi and Qwant, but remember to try them before switching.