A 48-year-old woman with metastatic colorectal cancer has completed 2 cycles of 5-fluorouracil (5-FU) and leucovorin. She now presents with severe hand-foot syndrome (palmar-plantar erythrodysesthesia), grade 3 diarrhea, and oral ulceration. Her performance status has declined. The oncologist is considering whether to continue, modify, or switch chemotherapy. What is the most appropriate immediate next step?