A 42-year-old woman presents with a rapidly enlarging dark nodule on her upper back that has grown over 3 months. On examination, the lesion is 1.5 cm, raised, dark brown to black, with an irregular border and surface ulceration. Dermoscopy shows a homogeneous dark pattern without distinct architectural features. Histopathology reveals a thick lesion (Breslow thickness 4.2 mm) with predominantly dermal and subcutaneous involvement, minimal intraepidermal component, and brisk mitotic activity. What is the most likely subtype, and which prognostic factor is most concerning?