A Botox and other neuromodulators (like Xeomin, Dysport and Jeuveau) can be used for slimming of the masseter muscles. The treatment typically entails 3-4 injections on each side, with 12-20 units per side, then reassessment in 2-3 weeks; more can be added if desired.
You can palpate your face and feel the masseter muscles on the outer lower jaw when you clench your jaw.
I am not sure how it would affect your jawline specifcally, but you are correct, filler can be used to further delineate the jawline. I'd recommend taking baby steps and getting an in-person consultation to better assess the situation.