I have made raspberry mead using 24oz raspberries/gallon in primary...turned out quite nice. I would not use EC-1118 for raspberry mead but would use 71B, D47 or even Côte des Blancs especially if goal is a 12-14% ACV mead. I also like K1V-1116 for a raspberry mead 15% or higher. Oh, no need to use three packets of yeast, worst case scenario would be two for a six gallon batch, but you should be able to use just one packet with no issues. I think the tea quantity is on the high side unless you plan on making nine cups of 'drinking tea' from your contributing water volume, three cups of drinking tea per gallon tends to work out well; but if you are adding dry tea to must I would drop it down to 1 tsp/gallon. I would not add brown sugar to this, simply not the right fruit profile but there is no reason you could not augment with some granulated sugar. Another option would be one container per gallon of Apple Raspberry concentrate (commonly found in freezer section at grocery). I would backsweeten with a simple syrup made from granulated sugar or the thawed concentrate.
Since you ideally want this finished two ways: dry and semisweet, I would construct this so that it finishes dry and then backsweeten a portion of it. That way you have the best of both worlds.
What ACV are you considering?