Answering to my own post :)

1. Icons were indeed missing. Installing kdelibs5-data seemed to fix the issue.
2. I just realized that you can open the CMakeLists.txt in Qt Creator, which parses the CMake file and opens it up neatly as a project. Learn new things every day!

So I now have the project debuggable in Qt Creator, which is what I was aiming for.

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