Water treatment only has an impact on the final beer with AG beers. All the water treatment will have beer done by the kit/DME manufacturer.
Your right, the reason why your 2nd/3rd beer has better body/head is because you used DME. Brewing Sugar is almost completely fermentable so leaves no body behind
yes, but here welsh water added something to the water a few years ago that leaves a film on cups of tea with no milk. So having a neutral baseline to start out from worked for me. The water is still soft but not a good as it used to be :-(