Good evening.
This is my first day with KDE after 9 years with Gnome.
So my first struggle in there is switching between 2 soundcards.
Here is quite nice feature that allows to control volume in all presented sound devices

But there is no point in that feature if you can't actually switch between these device "on the fly" by not clicking through many menus.

There should be checkbox or something that allows you do that comfortable.
Like in gnome-shell, see above

Is there any "systray-popup-checkbox"-style solution like Gnome-Shell and it's extensions offer?

You can move streams (Applications tab) between devices with drag&drop (drag the application's icon). There is currently no option to just select one preferred device and move all streams to it, as is in gnome.