For custom HIIT workouts, create a routine that prompts you when you hit BPM of X for Y seconds, the when you drop to BPM of A for B seconds. That's one interval, etc.

And then just hook your voice prompts into the system baked into Android already that has loads of customization.

Again, all of this is long-present in the Apple app. I'm already starting to regret my purchase...if I'd realized how much less dev time you give Android, I'd have likely found a different solution.