I think The brine was your problem. Next time kosher salt, black pepper and granulated garlic. Straight to the Grill char then put off to the side with the lid shut cook to 125, rest and slice.So we made the tri tip! I dry brined it and washed off the brine, let it come to room temp. We grilled it and it was cooked beautifully at 130F. Served it with roasted veggies and baked potato. The verdict... meh... really it didn't thrill me much and it reminded me of an eye round with better mouthfeel. It was a bit expensive too, $9 per pound. I think I prefer steak, but I'm glad we made it so now I now what all the fuss is about.
I have cooked thousands of pounds of tri-tip and the majority is very flavorful and tender.
A lot also depends on the amount of marbling in the meat, Always leave a small amount of fat on, don't trim it all off.
Nine bucks a pound is pretty spendy, Choice sells for 4 to 5 dollars a pound here.