A fleet carrier leading escort-carrier groups

Could Courageous or Glorious serve as aviation support ships later in the war?

They could perform some support duties, with the centre of an escort-carrier strike group as the more useful role.

The old carriers could perform some aviation-support functions like Unicorn, though a more operational role is also plausible. Their speed, air group, and command facilities could anchor groups of smaller escort carriers. This would keep them useful even when newer fleet carriers took the highest-intensity assignments. It also turns their command facilities into a fleet asset rather than treating the ships only as aircraft transports.