Wow, well this is a very interesting post. Frankly without knowing what “version” of Surya Namaskar you are “doing” I wouldn’t dare advise you specifically.
In the classical Surya Namaskar you move from Adho Mukha (down dog - which you refer to as 5) into vanarasana by stepping on foot up to a lunge - and then drop the back knee down or keep it lifted depending on the level of your current practice. Form there you step up to Uttansana, etcetera… so it’s tough to give exact breathing for you.
On top of that there are some different breathing methodologies again depending on how deep into your practice you are. Rarely does yoga come with one right answer. For one student straightening the legs is the appropriate action while another stiff or injured student should not be straightening at all.
more info, better answer.