Use an External USB SSD before logon (daemon)
Seems that MacOS has an option to do exactly what I need:
sudo defaults write /Library/Preferences/SystemConfiguration/autodiskmount AutomountDisksWithoutUserLogin -bool YES
Afterwards my external SSD is directly accessible by the daemon (if I wait some seconds because the daemon starts before the external SSD has been mounted). Additionally the external SSD is not accessible at all (independent of the user permissions). To finally get access I had to add my daemon (in this case the java executable) Full Disk Access in the MacOS security settings.