I’m pretty sure thread grafting a pine is not going to work (as has been stated by others numerous times already), but for the sake of the argument let’s say that you did manage to graft successfully to the bare interior sections. These bare sections became bare because the exterior growth/foliage of your tree shaded them, and without sun no viable buds developed right? If you are going to “maintain the size” as you say, won’t these grafts be shaded out, and therefore eventually fail because nothing has changed in terms of increasing sunlight to the interior of the tree? If you graft, sun is a big factor in getting those grafts to be successful long term. I’m having trouble seeing how you can both maintain the shape you have and successfully graft, I think you need to cut back and allow sunlight in if you want those grafts to be viable.
I should add, I’m no bonsai expert so feel free to disregard ...