Sounds like you found a good C for your son and that STBX is starting to be a bit more reasonable. Keep in mind that informal changes aren't going to be legally enforceable. That could be fine, just be aware of it.
I think the talk you had with your son sounds great, especially this part: "that nothing will make me happier than to know that s10 would have a good R with him."
Great idea to quit obsessing about someone else's life and someone else's R. You are the important one here
