Sleep and even Deepsleep is working fine now with the following settings:
Enable C6 in ACPI settings
Disable Siri (maybe someone can explain why this is neccessary)
Sleep works just fine with the bootflag -gux_no_idle, but i had problems waking up. In the Console.app i noticed that Siri is flooding it with events. Since I disabled Siri deepsleep is working like a charm and waking my system from sleep and deepsleep is not a problem anymore.
With deepsleep I mean: C6 state, all fans stop, all hard drives stop, my powerbutton starts flashing every second. To wake up i have to press the powerbutton once.
16GB DDR4 RAM - NVIDIA GTX 1050 Ti