I've made further progress, I eased significantly into the stroke. I'm more relaxed, I dive slightly deeper during the hand entry, I feel more in synch with my body undulation, said undultion is more relaxed and efficient.
My catch also feels much much better. Still a demanding stroke for me but Ienjoy now doing 25m (out of 50m) in fly and then finish the length using whatever stroke without feeling like dying.