I'd advocate IngeA's hints totally. You're in TI even as coach and really didn't run into that Problem before?
Torpedo and Superman are also drills to find out how slow
your legs will sink, when just holding them and your head aligned.
With your eyperience I think some tight looks into Terry's 2.o Mastery
, especially around the 2BK might be helpful. A well tuned 2BK will help lifting your legs too... and sinking legs don't sink so much in a single stroke-cycle around 1.1s... But after all, as Inge said working on for-back-balance most times will be more helpful, when getting it with into whole stroke...