How do I set up a different schedule at the week-ends to that of a weekday?

You can create multiple schedules for a profile and are not limited to just one. For example, you could create a schedule for Monday to Friday, and a different one for Saturday & Sunday (maybe with different timing/frequency), all pointing to the same profile.

However, SyncBack can only create (and then link to) one of them, so extra Schedule/s would need to be created manually via the Windows Task Scheduler. Provided you do not mimic the naming convention SyncBack uses, SyncBack will still link to the Task it created itself. If you do mimic the Task naming convention, which of the Tasks that SyncBack links to will be indeterminate. In either case, you would need to administer the Task that SyncBack does not link to manually via the Windows Task Scheduler's own interface instead.

Note that the fact SyncBack cannot 'see' additional Schedule(d Task)s does not  mean they will not work, only that you cannot administer them directly from the SyncBack user interface. Running the profile is the job of the Task, and it is not dependent on 'visibility' of the Schedule in question in the SyncBack interface, in the same way that a batch file, script or shortcut is not 'visible' to SyncBack, but triggering such an entity will still invoke SyncBack/the profile if correctly configured to do so.