MIL keeps asking me to supplement

We have this ‘problem’ since the beginning. I had c section and also I didn’t know how to latch baby so I struggled with bf from the beginning. I reluctantly supplement with formula while my MIL is over the moon to feed her formula, one of the reason is that she can feed LO as well.

As time goes on and I manage to latch baby well, I slowly move to ebf from 7 weeks. Now LO is almost 5m and is gaining weight steadily and my MIL is pushing for supplement formula again. She has 4 child and one of them gained 2kg a month from formula and she kept saying I won’t have enough as baby will need more and more as LO is growing up. She also said I don’t have that much milk bc my milk isn’t shooting out and my breasts arent firm.

I am open to formula if it’s necessary, but I dont need my baby to gain 2kg a month 🤦‍♀️ And also reason she is pushing is that baby has sucking motion while sleeping, is fussy when she holds her (stops fussy when I hold her tho). How do I establish firm but also gentle boundaries withy MIL because otherwise she is godsend (cooks amazingly, helps out us a lot with cleaning as well, she also tries to keep her opinions on many things that she doesn’t agree w us, but I guess there are just too many)?