An update from my side:
My setup has been correct..I found out that I didn't configure my iPhone and Android Smartphone correctly.
Modifying and using the iOS Profile you attached did however work!
Seems most OS don't easily support IKEv2 with PSK. Not even the Strongswan Android Client supports it (see "Known Issues" https://wiki.strongswan.org/projects/strongswan/wiki/AndroidVPNClient).