This is my own opinion:
Currently, I think you get the best end-user experience by using the desktop-qt5 part. This is a tutorial for how to do that: [Deprecated] Desktop App support - Qt5. If you’re targetting default Ubuntu, this option also has the best desktop integration; it changes the KDE theme to look like the Gnome theme.
After kde-frameworks-5-core18 has a global auto-connection, the KDE-neon extension will be a comparable option with some advantages and some drawbacks. This is a tutorial for the extension: The kde-neon extension
Advantages of kde-neon extension
- easier for the people creating the snap
- also includes KDE frameworks, not only Qt itself
- has smaller snap sizes and more KDE/qt-specific fixes.
Disadvantages of kde-neon extension
- less integration with the Gnome desktop; these apps use the default KDE theme when they run on a Gnome desktop.
The content snap from the KDE guide will not work for the same reason as the kde-neon extension. Imo, that guide is inferior to the kde-neon extension in every way; the extension basically does everything the guide tells you to do, but the extension does it better.