i finally got this working – altough it is not quite clear to me what exactly „solved it“.
I should have approached this a Little more „structured“ … anyway here is what I did:
• disabled WLAN „PowerManagement“ in my AVM Fritzbox DSL Router
• increased the voltage of my powersupply to 18 V (my original post was wrong – I only tried 12V – 16 V previously)
• disabled some WLAN Performance enahancement Options in my FritzBox (IP TV, automatic switching between 2,4 GHz and 5 GHz …)
• performed an update to the latest beta \ sw \ firmware
This image was the original basis for everything: m2p_rpi3_default_234_wps.img
Squeezebox with Spotify and Shairtunes work as well now.
I am happy
I am using iPeng 9 to „remote control“ the Squeezebox from my IOS devices