Hi @Stephane_Chevalier ,
regarding the missing information about the database upgrade, there is a bug ticket open for that SONAR-13125 .
regarding the systemd service definition i would suggest you run the sonar-application.jar directly as it is documented. the start.sh script will in the end call it as well using a java service wrapper, which does nearly the same as systemd is already doing just in a different way.
hope that clarifies things