She is totally healthy, puts on weight and is thriving
This is the thing that jumps out at me, She is totally healthy, doing all the right things, you are happy... sounds pretty perfect to me!

Apart from all the wonderful advice the other posters have offered, (convenience, money saving, time saving etc) can you tell DH/MIL this?? If it ain't broke don't fix it

You sound as if you are doing wonderfully by your DD, you persevered through the tough times and are now being rewarded by having a cruisy bf time, time to bond and enjoy together without having to sterilise, prepare, budget, think ahead. You do not need to worry about what your MIL wrongly suggests that
there is no way how to tell if my milk has all the nutrients DD needs and i never know how much milk DD is getting
because research has shown time and time again that our bodies produce exactly the nutrients our babies need and change and adapt over time to continue to meet our babies needs. It is a fantastic system and one which is working for you. You baby is thriving!!

You can always look at the World Health Organisation web site to find all the good facts and stats about breastfeeding or the Australian Breastfeeding Association. If you can arm yourself with the stats to back up what you already know and feel (judging by the things you have said in your OP) you may feel more confident to face the barrage of negativity.

Good luck, I am so sorry you are having a struggle with those around you, but as suggested previously, surround yourself with those who support you.