Just spit-balling here......if the deer like bedding there as it is, I wouldn't screw with it any more than needed. The deer more than likely like the knoll and the fact that they can see out, but anything getting in without being detected is very limited. If they are maple (sugar maple) they are very shade tolerant. I would consider hinging the outer edge of these to help thicken that perimeter. Simply cut an exit and entry place or two and be done. Planting conifers will require making sunlight available and many will not compete well without full sun. Ready for a concept that I'm going to get beat-up for.....plant some vining honeysuckle. Again it will need sunlight.....but hinge that outer edge and plant the honeysuckle (which is nearly an evergreen here) and manage it and it will grow a wall of screening cover. A buffer of greebriar or even black/raspberries could help as well. Biggest issue is these all need sunlight. The big thing is that the deer like being able to see out enough to detect danger while it's thick enough to hide them. You may not even need to go fully around the perimeter, maybe only half way would be sufficient as well. One of the things I have seen with deer bedding areas is they like to be able to have cover to their back with the primary wind blowing to their back, while they can see well in front of them. Something else that can help is cut those trees off about knee to waist high and they will side spout into almost like bushes as well. All just "ideas" to consider...nothing written in stone.