I make my own soil and you're pretty close to target. Technically imo, you're 50/50 because I consider everything else you listed as compost too. The only thing I buy is Sphagnum Peat Moss, Premium top soil and then the $1 cheap ass top soil. Mix 2 bags of the cheap ass top soil to 1/4 bag of the moss as your 50% balanced with your 50% compost. Once the bed is filled I'll put the premium top soil across the top, 2 bags should cover your bed when spread out. I get really good water retaining soil with this mix.
You can throw anything biodegradable in the bottom to occupy space. I use old firewood mostly, as it decays it will keep the bed warmer. You could also just go around your yard and find dead branches or prune your trees and use them as well. Newspaper, cardboard ( I'll even throw in old weed piles in the bottom and cover them with cardboard). You can either stomp it down, or put weighted pallets (rocks) on top of it for a week or so and let it get smashed naturally, so to speak.
I have the kitchens at a few of our hotels supply me with coffee grounds (big time benefit imo), and outdated fruit like bananas and apples (no oranges or lemons). Take my grass clippings from weekly mows, leaves from last fall, and use this method
This worked well in 6b, can't say how it would work in others though. You only probably need 6" of actually growing depth for most vegetables so you really only need 6" of soil, the rest can be whatever else you can throw in there to occupy the other 12"