You could refactor sshd so most network payload processing is delegated to sandboxed sub-processes. Then an RCE there has less capabilities to exploit directly. But, I think you would have to assume an RCE can cause the sub-process to produce wrong answers. So if the answers are authorization decisions, you can transitively turn those wrong answers into RCE in the normal login or remote command execution context.
But, the normal login or remote command execution is at least audited. And it might have other enforcement of which accounts or programs are permitted. A configuration disallowing root could not be bypassed by the sub-process.
You could also decide to run all user logins/commands under some more confined SE-Linux process context. Then, the actual user sessions would be sandboxed compared to the real local root user. Of course, going too far with this may interfere with the desired use cases for SSH.
But, the normal login or remote command execution is at least audited. And it might have other enforcement of which accounts or programs are permitted. A configuration disallowing root could not be bypassed by the sub-process.
You could also decide to run all user logins/commands under some more confined SE-Linux process context. Then, the actual user sessions would be sandboxed compared to the real local root user. Of course, going too far with this may interfere with the desired use cases for SSH.