RHEL9 was released recently. My syslog-ng git snapshot repo now also supports RHEL 9. I tested my syslog-ng packages on CentOS Stream 9. Let me know, how it works for you!
Note, that even if SQL support (libdbi) is enabled, the drivers are not there, I could not get them compiled. I plan to remove SQL support next week, as without drivers it's not much use.