here is a quick update.
@QuantumQweef : the theme has been shared by Wyn, not by me. If I misunderstood you though, just let me know and I will try to support.
1/ the kmix conflict indeed solved my issue regarding the volume button and the icon in the systray. Thanks!
2/ transparency issue with the open file window to chose a background image : well, before making a snapshot of the issue, I applied a few updates available and the kmix patch. I don’t knowwhether or not it is related but I cannot reproduce the issue anymore… I will monitor that and post a picture if relevant.
@karasu : I still use LightDM and not SDDM. So, no issue here.
In overall, I still have a messy desktop. I lost the update manager in the systray for instance (any idea ?).
Also, the system settings panel is in mixed language now (not a real issue for me but it is not clean). I don’t have anymore the Antergos “goodies” :) and the theme / window decorations I liked with KDE4. The netbook workspace is gone with KDE5 which I regret. However, if something equivalent to the homerun application launcher exists with KDE5, I can cope with that.
So, what should I do ? I am ready to start fresh and install again KDE4 after formatting my laptop. However, will that really bring the expected relief ? KDE4 end of life being at the end of August, does it mean I will have to deal with KDE5 in a few months from now anyway ? If it is the case, I prefer to keep working to overcome the current issues, avoiding by that extra installations.
I am seeking here for any advice ! Thanks !