Thanks, it worked.
I'm not sure what the issue is (no logs are provided which might give clues) but using autostart.sh should never be preferred over system.d, as the later provides better control over when things are executed.
The Problem is solved, but since I am here to learn I would like to also know why it didnt work before.
I have enabled debug logs and restarted my raspberry.
From the log file that I got I cut out the part where it is loading pulseaudio and attached it here.
I have found nothing else related to pulseaudio, custom.conf ( name of my file that is supposed to be used ) or /storage/.config/pulse-daemon.conf.d as a location within the log file.
If the provided cut of the log is not enough, is there anything in particular I should be looking for in the uncut log ?