Which procedure would allow me to achieve a more sharp and defined jaw line? (photos)

Much more information is needed, such as a full set of facial photographs including your neck from different angles in a chin neutral position. It would be also helpful to know your age. If you're considering losing additional weight, that should occur first before any surgical intervention. You also appear to have significant fat deposits located both above and below the platysma muscle in the neck.

Most plastic surgeons will most likely recommend liposuction as the gold standard for the removal of excess subcutaneous fat.
That said, it doesn’t look like you have a lot of excess fat in your neck.
Contour of this area is highly dependent on what the shape and size of the mandible is. having a projecting chin doesn’t mean you have an appropriate size mandible. The body of the mandible can be small with at the same time having a projecting chin. When the mandible lax sufficient length the underlying platysma muscle test draped in neck in the more oblique fashion. I’m not sure fat removal is going to get you the results you’re hoping for.
Personally I recommend liposuction if someone needs reduction of subcutaneous fat.
I don’t think your issues are related to access subcutaneous fat.
Consider having a few consultations with local plastic surgeons and oral surgeons. oral surgeons tend to see things from a bone perspective by plastic surgeons usually are more soft tissue oriented. Both specialties contribute to work in this part of the face.
Best,
Mats Hagstrom MD