* don't do the initial step where you almost boil the milk
* dissolve the powdered milk and starter yogurt using one of these [1]
* make the yogurt directly in the boxes ("tupperware") you'll be storing it in
* make large batches
[1] https://www.google.com/search?tbm=isch&q=sch%C3%BCttelbecher...
You can’t buy 1 gallon/4 litres of yogurt for $5 as far as I can tell. Maybe there is an angle I’m not aware of.
Plus you can be free of all the preservatives.
This is the speech of a scaremonger: You might not be trying to scare folks, but you've certainly bought into it. Preservatives cut down on food waste and are an overall good for society. More folks would starve without these and more folks would wind up with foodborne illnesses.
I'll mention that salt and sugar are preservatives, we've just found better methods now. Are all of them good? Nope, but food preservation was never perfect. That doesn't make it worse now.
No fearmongering intended.
There’s lots of natural preservatives. But we know that’s not always the case with most commercial options.