Basically you had a good start and it's what I do. But, I also slip a sleave of braided mono over the fly line first, tie core and mono together with some CA glue, then run the sleave over this connection. Whip finish # 8 tying thread at the ends of the braided mono, then paint with softex or aquatex. On the mono side you can tack the whip finish with CA glue to hold in place before you coat with tex. I can elaborate more a bit later if you would like. Flat mono is tuff and I am still perfecting that one.
Spey casters do it with longer rods