I have 11% dark malts in my stout using chocolate, roasted barley and black.
I have over 15% flaked products.
I don't have any issues with head retention and it drinks well without being harsh.
I think changing from a pils malt to something richer would benefit your recipe greatly. Maris Otter or a pale ale malt would work nice.
Also, on my stout, I don't use any acid in the mash and only acidify my sparge water to 5.4.
Lastly, Voss? Not a fan.
Based on your recipe, id use Apex London for a softer mouthfeel.
Good luck,
Brian