We can definitely look at supporting that if there's enough interest. Feel free to weigh in on the GH issue. Have you considered running a dedicated kube control plane for SpiceDB? This can be useful in the other direction: it keeps workloads on shared kube clusters from affecting your SpiceDB installs, which are typically more critical.
RBAC only protects your cluster so much from a poorly-behaved operator, and as mentioned in the thread victor linked, the UX of single-namespace controllers can be confusing for users, so namespace support has not been high on the list (but we can change that of course)