Place an oven rack in the center position, set a rimmed baking sheet on the rack below, and preheat the oven to 400°F (205°C).
Scrub the potatoes under cool running water, dry them completely with paper towels, pierce each 5 or 6 times with a fork, rub with olive oil, and sprinkle with coarse sea salt.
Place the potatoes directly on the center oven rack (do not wrap in foil) and bake for 60 to 70 minutes, until the skins are crisp and the center reaches about 205°F (96°C).
Cook the bacon in a cast iron skillet over medium heat for 8 to 10 minutes until crisp, then drain on paper towels, crumble, and divide into two equal portions.
Shred the cheddar and divide into two 3/4-cup portions, slice the green onions and divide in half, set out the butter and sour cream to warm, and warm the milk in the microwave for 20 to 30 seconds.
Remove the potatoes from the oven, rest for 10 minutes, reduce the oven to 375°F (190°C), and line a rimmed baking sheet with parchment paper.
Using oven mitts, slice each potato in half lengthwise, scoop the flesh into a large mixing bowl leaving a 1/4-inch layer attached to the skin, and place the empty skins cut side up on the prepared baking sheet.
Mash the hot potato flesh with the softened butter until mostly smooth, then gently stir in the sour cream and warmed milk.
Fold in the kosher salt, black pepper, 3/4 cup cheddar, half the bacon, and half the green onions until just combined, adding warm milk 1 tablespoon at a time if stiff; taste and adjust salt.
Spoon the filling evenly into the 8 potato skins, mounding it slightly without pressing down, and sprinkle the remaining 3/4 cup cheddar over the top.
Bake at 375°F (190°C) for 15 to 20 minutes until the cheese is melted and the filling is hot, then broil for 1 to 2 minutes for deeper browning if desired.
Top with the remaining bacon and green onions, rest for 5 minutes so the filling sets, and serve.