Light burn

If the lamp is burning too much and you can't raise it, switching to 18/6 won't really improve the situation with the burn. A lamp 20 centimeters from the plant is indeed very close. I would try to bend the top of the plant somehow, securing it in the bent position until the next watering, so it doesn't break during the bending. But it won't grow any taller, my friend. Its stretching period has long passed. As a last resort, you can take it outside into direct sunlight. Good luck, buddy! 😇👌