You can use the custom automations feature to automatically create subitems.
You can use an automation such as the one below:
However, this will be triggered by the creation of a main level item.
If I understand correctly, you’d like to have a subitem created when another subitem is marked as done.
It is possible to do this with the custom automations also:
It’s worth noting that in this case, every time a subitem is marked as ‘Done’ the same subitem will be created - it will not be possible to create different ones using this exact set up.
I wonder if you could have different status labels for each subitem you need to create?
You could then use the automation above and create an automation for each individual status label and specify the subitem that should be created.
Do you think this could work for you?
I hope this helps! Remember to mark as solved if this answers your question