What would be the best option for me to achieve a defined neck and chin angle at or close to 90 degrees? (Photo)

You are incorrect in assuming that chin augmentation would not improve your neck angle. A chin implant will not but a sliding genioplasty will as it pulls the muscles and skin under the chin forward. With your degree of horizontal chin shortness, a sliding genioplasty would be the best chin augmentation choice.

As a plastic surgeon, when I assess someone's face, I start off with, "would that catch my eye in public"?  In your case, you are a good looking young man with what I would assess as a minimal problem.  Now, that does not mean it does not bother you, but it probably does not bother anyone else.  You should have a discussion with a plastic surgeon about your body image.  That said, something minimally invasive like Ulthera may be something to consider if you had realistic expectations.  As the for the chin implant, that would not change the angle of your neck, but may improve your profile.