Aside from what you seem to understand about black water/grey water usage is maybe borax is not so bad after all:
https://permies.com/t/45332/add-borax-compost-soil
Maybe put the water in different spots each time to see what happens. If the plants seem to be affected then maybe mix the water with your dishwater.
If you are still concerned then use vinegar, lemon juice, or baking soda to wash the diapers.
My concern when I had a new baby was whether what I was washing the diapers in was gentle enough for the baby's skin.
Be sure to hang the diapers in the sun to sanitize them.