A 60-year-old man with septic shock secondary to an intra-abdominal collection has been treated with piperacillin/tazobactam and source control (CT-guided drainage). After 48 hours, his clinical condition has improved, inflammatory markers are falling, and vasopressors have been weaned. Blood cultures grew E. coli sensitive to piperacillin/tazobactam, amoxicillin, and trimethoprim. What is the most appropriate antibiotic strategy?