AFAIK, Vivaldi doesn’t really do anything to enhance privacy or security beyond a builtin adblocker and maybe a couple other things.
Currently, I have uBlock Origin installed and use DuckDuckGo as my default engine, so this should be pretty much equivalent of using Brave, right?
Not really. It is more equivalent to using a mainstream browser like Chrome/Chromium with duckduckgo search and uBlock Origin installed.
Like Firefox, Brave is a privacy-centric browser that does a lot of big and small things to improve privacy and in some cases security. Many of these changes are ‘under the hood’ (not things that are obvious to average users or visible in the UI. But it is those under the surface changes that often have the biggest impact.
A built in adblocker is a nice convenience and a sensible decision, but realistically that saves about 60 seconds of effort compared to installing a browser extension.
Vivaldi w/ adblocking and a private search engine might be a fine fit for you, for most of us privacy isn’t the only factor. But don’t mistake it for being a privacy-focused browser.
My impression is that Vivaldi is Chromium + more features, and a refreshed/creative UI. As you can see in the privacy comparison @davidcollini and @Sharply linked to, the privacy features Vivaldi has and lacks are virtually identical to Google Chrome, not to Brave or a well configured Firefox (represented in that test by either Librewolf or Mullvad)