If you’re targeting KDE runtime then you probably want to pull their dependencies directly using their repo, where they’ll be including the newer Qt toolkit as required by the runtime. Otherwise you’ll want to include an additional part which your main build depends on via after: [qt-part].
For example, for Gnome apps, I’ve done similar via enabling the gnome-3-26 PPA off launchpad.
Note that doing it this way will only update the build-packages, and you cannot use the packages in the ppa for stage-packages because those are fetched before the parts are started.
@kenvandine What voodoo do you do to bundle the libraries needed by snaps built using newer gnome libs?
Or do you only have libs staged in the platform snap?