This is the key log line. We do not have an ExecStartPre entry in 6.0.0 yet clearly your unit file still refers to one after the upgrade. This means you still have an old 5.x unit file and the upgrade did not replace the unit file with the 6.0.0 unit file. Maybe you edited the unit file (don’t do that!) and when you upgraded the package installer gave you a .rpmnew. You need to use the 6.0.0 unit file.
Try to test changes to your production system in a staging environment first.