Dear Bhante,
Thanks for letting us comment on an important sutta.
I like
But aren’t rassa and digha literally short and long? Why do you think it is idiomatic? Other than just long and short I’d prefer deep and shallow. Heavy breathing sounds like you’re out of breath. 
but but but… you know. 
a body among the bodies?
This sounds like you are practising breathing, rather than practicing experiencing bliss. I’d say: Breathing in, they practice experiencing bliss.
(Ow, I see Venerable @Brahmali has the same view.)
You mean they still the pitisukha, is that what you mean? From experience it’s the way you look at the bliss that you still, not the pitisukha itself.
I like this more than concentrating. 
I like your translation overall.