Congratulations mate! Seems to all be going pretty well at the moment
Why? Do you HAVE to do cardio? Are they not happy with your progress or is this some medical drivel?
Are you alternating HIIT with LISS on a every other day basis? or have I misread the above as 40 minutes HIIT split around 40 minutes LISS per day.
I've never found fasted steady state cardio difficult, HIIT on the otherhand is brutal fasted, especially if your carb macro is low.
That much cardio and so little calories, I'd be surprised if it spared your muscles completely. Are you doing weights too?