ok, late 60’s here, and I have COPD, low-medium, no O2 needed; use albuterol 2-3x/day.
Until I caught covid last year, I did (a kind of) low-medium level HIIT for 20-30 minutes, 2-3x /week.
One of the problems I found is exactly what they discuss, which is that it takes so long to to lower the heart rate during each rest cycle to even anywhere approaching normal that you start to wonder if you’re actually getting the benefit of HIIT.
Then just do the exercise as long as possible at the rate you actually can sustain. Don’t worry about the HIIT.