Non-binary breast reduction/mastectomy? Associated risks?

A  breast reduction that is as small as you mention would likely have very poor shape and very noticeable scars.  I would advise against it in most cases.

Thank you for your question and photos, I strongly recommend you consider a larger cup size as going too small may jeopardize vascularity and wound healing. Women no longer need to undergo massive breast reduction in order to experience pain relief. Please consider The Horndeski Method/The Bellesoma Method to alleviate pain from large breasts by transfering the weight of breast to the underlying chest muscles. This maneuver relieves the skin envelope in being the only structure holding breasts in place. The muscle acts as an active structure that easily maintains and lifts breasts long term. Attached is an example of a woman who wanted a lift without implants and without the ugly vertical incision. You are welcome to visit my gallery to see more examples. Best wishes, Dr. Gary Horndeski

What do you mean by "non-binary" breast reduction?  Binary means "consisting of, indicating, or involving two," so I must assume you are considering or desire the procedure on only one breast.  Regardless, because of your youth, because of the permanence of the operation, because of the magnitude of change you desire, because of the intimate relationship between your outer body and your inner femininity and personal identity, and because of the association of these elements with your future, you absolutely should embark on a course of self education beyond what may end up being misleading information you get online, information which may or may not apply to your situation.  See one or more surgeons in your area who are certified by the American Board of Plastic Surgery.

Your DD cup breasts are large and probably somewhat droopy.  Your best chance for an absolutely flat chest would be a double incision operation with the nipples made smaller and placed on your chest as nipple grafts.  The nipple grafts will have "protective" sensation but not fine, erotic sensation.  It would be helpful if you would post some photos of your own chest.Recommend you seek opinions from plastic surgeons who are familiar and comfortable with the transgender community.Elliot W. Jacobs, MD, FACSNew York City

Thank you for the question and congratulations on your decision to proceed with breast reduction surgery. You may be surprised to know that your goals/requests are not that unusual.   I see several patients every year who wish to achieve as small of a breast appearance outcome, for a variety of personal reasons.
 Generally speaking, it is possible to reduce the breasts size very significantly.  Based on the sophistication of your question, I think you may already understand that the concern with the amount of tissue removed is related to blood flow to the remaining tissue; if too much tissue is removed in one operation the blood flow to the remaining tissue (including nipple/areola) may be compromised. Part of the tissue that is left in place is called the “pedicle"; this segment of tissue is responsible for delivering the blood supply to the nipple/areola tissue. If the pedicle is made too small (in the effort to reduce the breasts as much as possible) then patient will likely have problems with tissue survival.   Occasionally, a staged procedure is helpful.
I think that you are wise in understanding the importance of preoperative communication when it comes to achieving an outcome that you will be pleased with. In my opinion, successful outcomes with  etc. surgery depend on:
1.  Careful selection of plastic surgeon (and from the surgeon's  standpoint, careful selection of patient).  I would suggest starting with the American Society of Plastic Surgery and/or the Aesthetic Society of Plastic Surgery to obtain a list of well experienced board-certified plastic surgeons. Then, I would suggest you visit a few surgeons whose practices concentrate on aesthetic surgery. ***Ask to see lots of examples of their work and preferably speak/see patients who have had similar procedures done.
2.  Careful communication of goals ( which I will discuss further below).
3.  Skillful execution of procedure ( preoperative, intraopererative, and postoperative patient management).
In my practice, I would ask that you NOT communicate your goals, or evaluate the outcome of the procedure performed, based on cup sizes.  There is simply too much variability when it comes to bra sizes between bra manufacturers and even store employees doing the bra fitting measurements.
 Generally speaking,  for the benefit of patients undergoing breast reduction surgery:  it will be very important to communicate your size goals with your surgeon. Most patients wish to achieve a enough of a reduction to help with their symptoms while remaining proportionate with the remainder of their torso. Again, I suggest that you do not communicate your goals in terms of achieving a specific cup size. For example, a “A or B cup” may mean different things to different people and therefore may be a source of miscommunication.
In my practice, I ask patients to communicate their goals with the help of  computer imaging and/or goal photographs,  as you have done here.  Obviously, the outcome associated with the breast surgery will not necessarily match that of goal photographs perfectly, but they do serve as a better communication tool that subjective terms such as "natural", "proportionate", "flat"… Evaluating goal photographs also allows for a plastic surgeon to determine the consistency of the patient's goals and allows for a discussion of realistic expectations as well.  
***Needless to say, when it comes to achieving patient satisfaction with the outcome of surgery,  it is very important that a patient has consistent goals (fully decided on what she would like to achieve) and a good understanding of realistic expectations (what outcomes can and cannot be achieved). Some of the risks/potential complications associated with breast reduction surgery include:  infection, bleeding, seroma, hematoma, wound healing problems, abnormal scarring ( hypertrophic or keloid),  loss of sensation, inability to breast-feed, breast asymmetry, necrosis of tissue or skin,  unsatisfactory cosmetic results, unpredictability of exact cup size postoperatively,  recurrence of the breast hypertrophy, and the potential for further surgery.  Other risks  related to surgery in general include deep venous thrombosis (clots),  pulmonary embolism, pneumonia  and even death.
 Best wishes with your decision making and for an outcome that you will be very pleased with.