First, SES 7 comes with SLES 15 SP2, not SP3. Not sure if this would cause any troubles, but it might when you want to register them for the SES add-on.
Second, all minions need to be able to resolve the Salt-Master’s hostname. Did you verify that? A working DNS is helpful, but a complete and correct /etc/hosts file is enough in a lab. If you don’t want to or can’t disable the firewall the docs tell you which ports to open in order for salt to work.