hey, thanks for your replies 
“which OpenGAPPS” is not really an argument, is it? As soon as you install even Pico, you will have the full google-calling-home package (Play Services) – won’t you? That’s where my original question comes from. The only reason I see to go OSOS is to get rid of google.
(+ easy way to get root – anything else?)
You do need it for Signal for example :\
And if you say you install, e.g. WhatsApp from APK – who is doing the push-service for you? (Fairphone?)
Thanks again – more input is very welcome 
ps, also: will you have to do this everytime you update? @freibadschwimmer