In my experience it sounds like you are managing to express a huge amount! I had the same oversupply issues. I was EE my DS for four months as he had a heart condition and was in the hospital fed through a tube for a long time. The nurses used to joke that I could feed the entire ICU! I was getting about 200 mls each pump and would pump every three hours.
I agree with not overstocking your freezer, I ended up with a huge amount stored in my freezer as we would go long periods with him not having much or any milk at all due to surgeries etc but of course I had to keep pumping for supply. I was lucky enough to be able to donate it but I can imagine how devastating it would be to have to dispose of it. he night of his final surgery I was so stressed I had a couple of drinks and then "pumped and dumped" my next pump. Even just tipping that little amount down the sink broke my heart.