A 58-year-old woman with newly diagnosed multiple myeloma (IgG-κ, 35% plasma cells, standard-risk cytogenetics) has completed 4 cycles of bortezomib-lenalidomide-dexamethasone induction therapy. Serum M-spike has decreased from 3.2 g/dL to 0.8 g/dL. She is 58 years old, has normal cardiac function, and is deemed fit for transplantation. What is the most appropriate next step in management?