For the ref my situation has greatly iproved, the bulging disk might still be there (their disappearance is rare) but the herniation is gone.
My neck mobility makes strong progress BUT I realize I've to be careful with regard to breaststroke and butterfly alike.
I still made some progress in fly. Like with all the other strokes I realize I was putting to much energy in the catch more than I could sustain.
Following the advices of swimmer I met I eased my undulation: I was overdoing it (undulation and kick).
I still found the butterfly catch and overall arms movement complicated especially as it seems there are significant different approach from Phelps perferct diamong to wider catch followed by almost straight back ward pull and push.
I start to lean toward the "diamond" approach and doing it well which in my case means sticking to a pretty narrow catch. I lack the strength to catch wider and then pull back thewater and my hand together at the right timing.
I'm leaning toward the diamond shape because it sets me for a good recovery at the cost of some power lost.
Still a work in progress... I'm not relaxed enough, my breathing is not ok, etc. but I think I have somewhere to go: relax and stick to a given technique. I'm still unable to break 50m but I believe the breakthrough is close.
PS: I've made quite some progress till I practice in a 25m pool.
1) it is less busy than the 50m and butterfly is not exactely people friendly.
2) 50m is a long distance to butterstruggle through... lol
3) there has to be a CON... my turns no matter the stroke... it kicks me out of rythm it affect my front crawl significantly but kills my butterfy.