Khurasan’s SEAL Team 6 are real world/modern, but have a similar look to the Galactic Grenadiers in your second link. They also have separate/optional backpacks, so you could add a more sci-fi looking backpack if you wanted. They are 15mm, but I can compare them size wise to an 18mm figure if you want.
Those are brave men knocking at our door. Let's go kill them!