I really like how you fixed those tips with that plug/spice/overlay. I have the exact problem you are having with some woods, but not other. For me, Ash is just imposable to recurve, it splits open and the curves you do get pull out and elm likes to open cracks. Osage and mulberry are sweethearts to recurving. Those are sharp curves you are doing there. I know in ship yards that build wooden boats, they steam the wooden ribs, use a large metal strap as a back for the rib, and bend and clamp them to a caul. The strap keeps the back of the curve from opening up. You should try something similar?