So I am not sure if I did everything correctly ... I added --all-scopes-completion to the clangd options (verified its running with them) and have code like:
namespace Foo { struct BarXXX {}; }
Now when trying to complete BarXXX it shows up as [ID] but not as Foo::BarXXX as I would expect from what I saw with other tools and posts (e.g. clangd/clangd#1082)