What constitutes a natural burial? At a cursory glance there are only about 5 states that don’t permit home burials and many of those just say it has to be in a cemetery, but you can apply for a family cemetery on your property and it’s completely legal.
In Virginia and West Virginia at least there are no requirements whatsoever that you use a casket or bury them to any specific depth. I’d suspect that if you were disrespecting grandma and threw her in the garbage you would be breaking desecration of remains laws but doing a legitimate burial at home is completely fine.
I can only speak to the laws of my state and those around me, and I suppose local municipalities might have differing laws, but it’s pretty open ended. You do not need a funeral home involved at all and frankly given how expensive these things are I totally support families that go that route.