.
Even if your mnilk is somehow ‘tainted,’ it is less tainted than soy, which in unfermented form is toxic to every mammal.
This is true of most other beans too. They need to be soaked and washed to eliminate the enzyme inhibitors that stop digestion.
Are you against ALL formula or just “soy-based”? In my experience as a mother who is friends with lots of other mothers, the only mother I knew who was using the soy variety had a baby with severe digestive issues.