that reference is a challenging piece to pull anatomy from. They aren't showing bones (or even landmarks like the spine of the scapula or the PSIS) and they are grouping some of the muscles together. Frankly, I'm impressed by your effort of getting the simplified skeleton in there (seriously! that's a challenge). It's difficult to critique this because I don't know where you are on your anatomy journey but my impression is that you need to find references that show both the bones and the muscles and focus on smaller muscle groups before attacking the whole back.