How to create a big enough breast milkstash

I’m seriously struggling. I can produce anywhere from 2-4 oz per pumping session. My baby is 6 weeks old and taking 3 oz per feed and he’s eating every 2.5-3 hours. I pump and pump but have been supplementing with formula for weeks now because I feel like every time I get enough pumped, I fall behind. I noticed if I pump more frequently I actually make way less. Makes no sense, I know, but it’s true 🤦🏼‍♀️. I try for every 4 hours but what I pump ends up in the next bottle and it’s gone. How do women create stashes?!? I just wish I could be far enough ahead to not worry because on the days I pump less, it’s not enough to feed my baby or create a 3 oz bottle. Tips? 💙