There are two schools of thought on early addition of sulfite to white
wines. One camp feels it is best not to sulfite must as the browning
actually falls out as the wine clears leaving you with a wine lighter
in color and resistant to browning since the reactions already
occurred. The other camp uses 50 to 100 PPM at crush based on health
of the grapes, moldy grapes get the higher dose. I sulfite but have
tried the no sulfite avenue too.

Washing grapes - especially if you are depending on natural yeast, is
not recommended. Cultivated yeast is cheap, predictable and reliable,
using natural yeast could result in a batch not finishing to dryness
but it seems to be fine with your wines.

Measuring sulfite levels for red wines requires both a decent
calibrated pH meter and an aeration oxidation apparatus; that will run
approximately $300 so if you are not looking to jump into that water
it's understandable. Titrettes from Chemetrics work fine on whites
and are relatively cheap.

Your first rack should be with some splashing; after that racks should
gentle with no splashing. If you follow that process you can probably
just add 1/8 level teaspoon of potassium metabisulfite per 5 gallon
(US) to the wine each rack and will be safe; that is around 20 PPM.
As mentioned by others, sulfite additions are governed by pH, higher
pH requires more sulfite so a good pH meter is worth having. Since
you mentioned your acids are not high you might need 1/4 teaspoon, at
least at bottling. That is where the pH measurement would be helpful,
a local school's sciences department would have one if you don't want
to do that.

Joe