I assume your title is an error?
Oh, well...
The treatment is for you only - at least at this point. A treatment helps to understand what you're writing about. It's a great guide. So it makes more sense to write down the details before you write the screenplay. That said, if you want your treatment completed for future reference , go ahead and go back to it once you're done writing the script, and rewrite. Things change during the writing of the script, so you may need to change the details in your treatment.
A tip, though: Write an outline before you write the script. Just as important when it comes to understanding what you're writing about.
EDIT:
If you're going pro, agents aren't your goal, but managers. Make sure to do your homework, because it's not simple to land a rep and/or sell. Also, if you're a novice, you're light years away from starting to look for representation/trying to sell. So make sure you bring yourself and your skills to a professional level before you become a pro. And that includes leaning about the business side of the craft. It wouldn't work otherwise.
And lastly, be careful with the feedback you've been getting. Good feedback is not necessarily a good thing. It could be fake (to be nice/not to hurt your feelings) or it could be from people who aren't professionals and don't know much about the craft and the biz. What I'm saying is, in the real world, execs and pros - even the buyers themselves - are much tougher on you with the drafts, to a degree where it could get horrible. I don't know who those people are that you trust, but I'd get an opinion from a pro who's objective and doesn't know you. What concerns me the most is that you're a novice, and that exposes you to many beginner mistakes, naturally. And your drafts would show that as well.
So I suggest that you don't rush to get representation/sell, because you'd be wasting your precious time. You don't land representation/sell after one beginner-level script. It take people who do have professional skills and samples years to land a rep/sell - if at all. Don't assume it's that simple, and take your time to master the craft.