Combine flour and 2½ cups room-temperature water in the bowl of a stand mixer fitted with the dough hook. Mix on low speed until a shaggy dough forms. Remove bowl and cover with cloth.

Stir yeast, sugar, and ½ cup warm water with a fork in a bowl to dissolve; let sit until yeast is foamy, about 5 minutes.

Pour yeast mixture into stand mixer bowl and mix on low speed until dough absorbs the water, about 1 minute. Add salt and continue to mix, increasing speed to medium, until dough is extremely elastic and very sticky, about 5 minutes.

Pour 3 Tbsp. oil into a large bowl and coat sides. Scrape dough into oiled bowl. Cover and place in a warm spot until dough is doubled in volume, 2–3 hours. 

Drizzle 2 Tbsp. oil over a 18x13" sheet pan and rub all over bottom and sides. Fold dough inside bowl a couple of times to deflate, then scrape onto prepared baking sheet. With oiled hands, lift up dough and fold over onto itself in half, then rotate baking sheet 90° and fold in half again. Cover dough with a piece of well-oiled plastic and let rest 10 minutes to let gluten relax.

Uncover and oil hands; gently stretching dough (to avoid tearing) across length and width of baking sheet in an even layer, working all the way to edges and into corners. If dough starts to spring back, let sit 5–10 minutes and start again. Cover again with same piece of oiled plastic and chill at least 8 hours and up to 24.

Let sheet pan sit in a warm spot until dough is puffed and nearly doubled in height up to the very top of the sides, 45–65 minutes. Meanwhile, place a rack in center of oven; preheat to 450°.

Drizzle dough generously with more oil. Oil hands again and press fingertips firmly into dough. Sprinkle dough with marinoberries. Push dough and berries down all the way to bottom of pan to dimple all over, allowing the berries to incorporate into the dough instead of laying on top. 

Mix 1/2 tbsp kosher salt and sage with 2 tbsp warm water. Pour mixture over dough, filling dimples. Sprinkle generously with sea salt.

Bake focaccia until surface is deep golden brown all over, 25–35 minutes. Let cool in pan 10 minutes. Slide a thin metal spatula underneath focaccia to loosen from sheet pan and transfer to a wire rack. Let cool completely before cutting.