A 38-year-old woman presents to the ENT clinic with a 2-year history of a painless, slowly enlarging swelling in the left submandibular region. On examination, the mass is mobile, non-tender, and measures 3 cm. Intraoral examination shows a smooth bulge on the floor of mouth. MRI reveals a well-defined, heterogeneous mass with areas of high signal intensity on T2-weighted images. FNAC is inconclusive. What is the most appropriate next step in management?