The first sinew backed bow I ever made was a short osage/sinew bow that looked a lot like what you've got there. Mine was too stiff in the fades as well, so I tried to get it working more there and it ended up dropping the poundage way more than I wanted.
I am no expert when it comes to sinew. The one observation I've made though is that it's important to... take a little bit off, then bend.... Take a little bit off then bend... Take a little bit off then bend... until you get it close to where you want it. It's tedious, but it's way too easy to go from not bending enough to bending way too much.