A 19-year-old Aboriginal woman from a remote Northern Territory community has a history of acute rheumatic fever at age 11 and established moderate rheumatic mitral regurgitation. She has been receiving benzathine benzylpenicillin G 1.2 million units intramuscularly every 4 weeks but had a recurrence of rheumatic fever 12 months ago. She is otherwise well and wishes to stay in her community. What is the most appropriate long-term secondary prophylaxis regimen?