Between these two options DuckDuckGo Browser is at least free and open source, while Vivaldi is closed source, which makes DuckDuckGo Browser the better choice.

Firefox and its forks are better than both. Firefox’s Gecko engine is independent of Google and Apple, while Vivaldi uses Google’s Blink engine and DuckDuckGo Browser uses either Blink or Apple’s WebKit engine depending on platform.

Direct link to PDF of RCS Universal Profile specification

This PDF link can be found on a search engine. You can also fill out the form with fake information and a throwaway email to get the link, though you don’t have to.

The RCS Universal Profile is a publicly available standard, but there is no FOSS client stack that implements the standard. Google restricts RCS support on Android to the proprietary Google Messages instead of making it part of the Android Open Source Project. Apple Messages and iOS are also proprietary.

The EU should use the Digital Markets Act to force Google and Apple to allow competing RCS clients on Android and iOS.