I personally cannot do anything high intensity immediately after dosing ECA, whether it be cardio or training (I feel woozy, light headed and wanna puke ><).

If I cannot train first thing in the AM, I find ~5hours after my last dose to be my cutoff point to avoid above symptoms. That being said I take my doses as close to 5 hours apart as possible.

On off days (aka cardio only days), or nights when I'm doing PM cardio, I do dose ECA immediately beforehand. I do not do HIIT like you. I keep it low intensity, 60-70% of my max heart rate for 45-60min, typically on the elliptical. This method allows me to keep strong through the duration of my cardio session and avoid feeling like shit. It also makes me sweat bullets. I'm extremely mindful of my water intake when I do this.