Many DEs would require setting QT_QPA_PLATFORMTHEME=qt5ct (and installing/using that qt5ct, that's a GUI configurator).
Some KDE apps and libraries are interdependent on upstream versions of each other. This means, to develop e.g. KMail you'd have to get the upstream version of it. There're multiple way to deal with it.
ATM it's implemented terribly: you need to actually "install" stuff somewhere, and then export a bunch of stuff. You can't just do "make run kmail", thus making it to use everything you just built.
"Building the world" happens through kdesrc-build, see examples for kmail development here (including the exports): https://community.kde.org/KDE_PIM/Development/Start#Compiling
kdesrc-build is probably bad in handling dependencies, as in my limited experience it haven't figured that kmail depends on upstream version of QtWebEngine, and I've been told on IRC that one indeed have to get upstream QtWebEngine some way around kdesrc-build (e.g. as a package). In addition, upon asked to build kmail it went building 1xx apps, most of which have nothing to do with kmail; and worse of all, it failed to build 5x of them. So yea, it's definitely bad.
It's a distro with all that latest stuff. A docker image specifically for development PIM stuff (which is the one most often requiring upstream versions) can be found here https://community.kde.org/KDE_PIM/Docker