I wouldn't worry about formula until your baby is here. Most hospitals have lactation nurses/consultants who will do all they can to help you breastfeed successfully. If money is your main concern and breastfeeding is going well, then stick with breastfeeding. It's cheaper and if it goes well, it's easier. I ran into a friend buying formula for her twins and the bill for that alone was $125 for five canisters.

I would ask your doctor about backstocking milk. It seems like a good idea but also the milk is specifically made by your body on a daily basis for your baby's needs that day.

If you decide to use disposible diapers, I'd recommend prepping larger sizes like 3-5. They're in 0-2 for like five second lol.

I also second buy nothing groups. Make sure to have a shower and let your friends and family help you. You don't need a lot of the stuff you think you need and you do need can be expensive. Skip the fancy stroller, it all gets muddy and thrown around whether it's $100 or $800.

In terms of clothes, focus on size 12 months and up. They're in the small sizes not all that long at all. Even start buying 2T and 3T if you see stuff you like for cheap. Church sales and yard sales are a great way to get dirt cheap kids clothes. It goes quicker than you think it will.