Definitely buy bits each month and plan out your most expensive items so you know when/where you're getting them from.
Agreed that Facebook marketplace is amazing and eBay!
Breastfeeding has saved me tons of money that I would otherwise use on formula (if you can do it, not for everyone or not possible sometimes I know).
Don't buy an expensive breast pump unless you absolutely know that you'll need it. I learnt this the hard way after I bought one in a panic, then ended up building most of my freezer stash with a £15 Hakka anyway!
Clothes wise, if you've got a particular shop in mind that you feel you absolutely have to get clothes from that you know is more expensive (H and M/Zara/Rivers/Next), if you can, wait for the sales especially Zara and Next sale can be amazing if you time it right. And absolutely do not waste money on baby bodysuits or sleep grows from there as no one will see them hardly!
When weaning, buy cheap silicone ice cube trays to store puréed food if that's what you're doing and buy fresh and plan a day of food prep to make a couple weeks worth so it's ready. So so so much cheaper than buying jars and snacks from out
Aldi nappies are amazing and so so cheap. But if you've got the money available, do invest in cloth nappies but make sure you research the ones you want and feel you can manage thoroughly.