What's the best treatment for lacking facial/jawline definition and a rounded chin?

It would be helpful to see pictures of your face to provide a more qualified answer. Some form of bone augmentation is almost always best, whether it just chin implant augmentation, chin and jaw angle augmentation or a more complete wrap around jawline implant.

generally, the best way to define the facial structures is to ensure the underlying bone support is well balanced and appropriate in positions. So if the chin and lower jaw are underdeveloped, the middle face appears 'round/full' and there is no good separation between the neck and the face (ie jawline). So focus on that analysis first, perhaps with a consult early on. The use of liposuction to 'reduce fullness' is unusual except in the area under the chin and only if the chin is well proportioned. Fillers can 'imitate' good bone support for yearly expensive injections will surpass the cost of doing it properly