I find myself quite convinced that in the ages after Melkor’s defeat, Mairon was not trying to conquer Middle-earth simply for his own sake; he was trying to bring Melkor back.
“But Manwë put forth Morgoth and shut him beyond the World in the Void that is without; and he cannot himself return again into the World, present and visible, while the Lords of the West are still enthroned.” - J.R.R. Tolkien, The Silmarillion
“…while the Lords of the West are still enthroned.” Middle-earth is only a stepping stone on the path to victory. Once it falls to him, once he gains a firm hold upon it and readies his armies once more, he will march against the West to overthrow the Valar. If he can overcome them, he can be reunited with his beloved Master. He can bring Melkor back into the World that Is, and he can lay that World as a gift at Melkor’s feet. This is Mairon’s endgame. Of that I am convinced.