Plasma-pa needs an option to show per-application volume level sliders without requiring the user to right mouse click on the plasma-pa speaker icon and then click on "Audio Volume Settings". The per-application volume sliders are used very frequently by many users, and in KMix they can be accessed with one click by clicking on the speaker icon in the tray. There also is no setting in plasma-pa for adjusting the "Event Sounds" level, whereas there is in KMix. plasma-pa also has no option for vertical volume sliders, which many users prefer over the horizontal sliders. Once feature parity is complete, my suggestion would be to drop KMix entirely as it currently segfaults on Qt 5.6.1, and shift all development to plasma-pa. But as I wrote above, plasma-pa needs more functionality before we can drop KMix. Reproducible: Always
Please try with Plasma 5.7, most of the things you listed should be fixed now. Also after that, please open one bug per issue.
fwiw, neither 'event sounds' or 'vertical sliders' are supported (yet), but I agree, one bug per issue/rfe
Event Sounds (displayed as Notification Sounds) are supported in 5.7, but it is displayed only in KCM. Vertical sliders are not supported, but in current state it doesn't really make sense as popup of systray applets cannot be resized by the applet, and it would require the possibility to grow the applet width (as kmix popup when using vertical sliders).