Hi,
Definitely no need for formula. Also, don't feel you need to replace all breastfeeds with milk. They just need food at this age, including calcium (which they can get from other things as well), so milk can just be a part of their diet, not a major component of it.
The ABA will have advice on how best to go about weaning too

I'm not sure what kind of drugs you need to take, but you might find THIS THREAD interesting. I didn't wean when I did the IVF cycle my DD was conceived on.

All the best!