Lowering the alcohol in a kit , probably not a good idea as it will throw it out of balance.
I'm wondering if the kit really is that potent as most are geared for 12.5 percent which is lower than the common 14% plus of commecial wine.
if its a taste thing and the wine tastes hot let it age more , if its a head wobbly thing some tips.
try the mist kits , they are much lower in alcohol.
or make fruit wines where you can set the sugar levels
use smaller glasses .
when I'm having a session with other winemakers it can get a bit heady , so I always make sure I have a full stomach and I often have a pacer glass of water between glasses of wine. a nice bottled water on the table and some nice glasses is actualy a classytouch and will help everyone. every professional tasteing I have been to does this.
the blackberry addition you did would only add sugar and make the wine stronger.
when all else fails , drink less, everyone metabolises wine at differnt rates.
my brother in law can drink beer till the cows come home but 3 glasses of wine and he's a mess.