QEMU 6.2.1 is from 2021. Like I mentioned above, if I found the right patch above, SMP support for SPARC has been added 2024. So I think you need a newer QEMU. From the release history, I would expect that you need at least a version 9.0.0.
If your distribution doesn’t provide a current QEMU, you can just build one from the sources. Usually QEMU works quite stable in the development versions so I would just use the latest git version.