Is it possible for Core to auto create folders when saving files using a variable from a manifest? i.e. if the folder doesn't exist, create it.
That's not part of the functionality in the Save component. However, it is something that should be implementablewith some scripting.
When you build the workflow add an external node before the save node and pass the save location to a script. FFCore makes a CLI call to execute the script so the script can be written in your language of choice.
The script would parse the defined location and create folders as needed. This should be reasonably simple to do. However, if you need help with implementation please contact your local account team so they can engage the Solutions Engagement Team. They do this sort of thing regularly.