What documents do I need for D-4 visa extension?

My D-4 visa expires in about 6 weeks and I need to extend it for the next semester. I've heard the process has changed recently and I want to make sure I have everything ready.

From what I understand, I need: passport, ARC, application form, enrollment certificate, attendance certificate, bank statement (showing sufficient funds), and tuition payment receipt. Am I missing anything?

Also, how much money should I have in my bank account? I've seen different numbers online - some say 9 million KRW, others say less. And do I need to book an appointment at the immigration office or can I just walk in?