What is going on with my weak, undefined neck/jaw? (photos)

Your pictures are not very good to evaluate your concerns. But chin implant does not seem to be a necessary option. The angle of your neck indeed could be improved but you should have a hands on consultation with a PS.

I do not see any issues with your neck and chin. You have a very nice chin and I would not do surgery on it.

Chin implant surgery has long been touted as a simple operation that reliably gives good results. The result may be compromised by the preoperative analysis, the surgical technique, the implant design and the host response to the implant material.The adequacy of chin projection should take into account several variables including sex, the effect of occlusion on lower lip position, the thickness of the chin pad, and the depth of the labiomental fold. When these variables are not considered, implant augmentation may result in a chin that is unnatural and too large, particularly in a woman.