Whisk tapioca flour and cocoa powder in a medium bowl until no lumps remain. Heat chocolate, oil, and 1 Tbsp. tahini in a small saucepan over low, stirring, until melted and smooth.

Using an electric mixer on medium-high speed, beat eggs, swerve sugar, and coconut/brown sugar in a large bowl until light, smooth, and doubled in volume, 3–4 minutes. Beat in salt and vanilla to combine, then beat in chocolate mixture, scraping down sides as needed, until incorporated. Beat in tapioca flour mixture, then increase speed to medium-high and beat until mixture is thick and holds its shape, about 30 seconds.

Stir maple syrup and remaining 3 Tbsp. tahini in a small bowl. Scrape batter into prepared baking dish and smooth top. Dollop agave mixture over and swirl into top of batter with a skewer or toothpick.

Bake brownies until sides are puffed, top is browned, and a tester inserted into the center comes out with a few moist crumbs attached, 22–26 minutes. Let cool before removing from pan and cutting into 16 squares.