It is hard to see in there. If you do any thinning, be sure to use very sharp shears and get in just above a bud and nip it off cleanly without cutting through any of the needles. You can use a chopstick to poke around.
There might be areas that need to thicken (I really can't see) but say if the apex needed more thickness you wouldn't prune as much there. My needle juniper grows almost all year, even in the wintertime it seems to be pushing growth. I have to go in and both prune back to shape the branches and then thin it out too or it will get
congested
This diagram may help. There is a strong bud at the apex that would be a logical place to prune back to, but that's assuming that the apex doesn't need thickening, which is something I can't assess from these photos.
thinning
I do know that if you don't let light and air get in there there is the danger of dieback and spider mites and so forth. Prune back cleanly to live buds and you'll be okay. If you do that it's very unlikely you'll do any damage. If you prune back arbitrarily, especially when cutting leaves, it is likely to die back to some other branch. As someone else said, get the moss off the trunk. Organic fertilizer would be a good choice.
I don't think this needs to go into a grow box as I believe was suggested earlier, but it is hard to see the condition of the rootbase.
Regards,
Matt