If you have the Play Store … How about Lux?
https://play.google.com/store/apps/details?id=com.vito.lux&hl=en
I don’t use it myself, but that would probably be my first try if I would see this issue as a problem so vehemently.
Edit: I just installed the YAAB APK from
https://bitbucket.org/gyrussolutions/yaab/downloads/
… and it seems to work as intended.
I’m on LineageOS, which currently is based on Android 7, which Fairphone Open OS will be based on soon.
Edit 2:
Can you check whether YAAB has the permission to “draw over other Apps” (is called differently in other languages)?