Breast milk to formula spit up

Help!!

I dried up, and baby had a ok latch but I only lasted 4 weeks with breastfeeding

Anyway we switched to formula. She isn’t a fan and takes forever to even like the nipple in her mouth. She did have a nipple with breastmilk so she was already used to bottles.

She was eating 4 oz of breastmilk and now she barely chokes down 2oz. 3 if I’m lucky.

Should I be concerned

She is peeing and pooping. Not as many poops but she does poop. She doesn’t seem super gassy and she’s a happy baby but she isn’t eating much.

Should formula fill her up more than breast milk?

Also is there a probiotic I can give her to help her tummy just incase?

It’s only been 5 days with formula will it just take time?