Cabbage leaf definitely helps, if you feel some lumps you can massage and hand express to remove the lumps. I think 20 min might be a bit long if you want to reduce supply, you can stop when you see the milk is not coming out or coming out very slowly. For me that only took less than 10 min
you can just pump abit instead of clearing your boobs fully. this process will takes a few days and you might feel engorged. you're trying to send signal to your body to produce lesser milk.
putting cool cabbage leaves on breast before a bra.
Sleep with cabbage leaves. :)
mel