I would like to automatically 'toggle' data by time

I have a data set which includes a yes/no variable (Available)

The workflow changes the ‘Available’ date piece to ‘no’ during a workflow - all ok !

How would I get this to ‘toggle’ to yes automatically after 30 minutes ?

Create a backend workflow to change the value to ‘yes’.

Then, from within your first workflow schedule the backend workflow to run for the current date/time plus 30 minutes.