Hi Tom, congratulations on your recent success, I can almost feel your elation from here! You have overcome an obstacle that is still eluding me at present.
I too was stuck on 50m for a long time & now 75m is my limit before a length or two of backstroke is needed to recover. I began TI almost a year & a half ago, worked diligently through all the drills, even had some one-to -one lessons with an instructor who practices TI freestyle, yet despite all this relaxed breathing still has not fallen into place.
Lately, at least once a month I feel like throwing in the towel but thankfully people like you take the time to post their achievements & this is what keeps me going.
Thank you for sharing your insights, I am looking forward to trying out your method at the pool later.