^^ Totally agree w/ Lisa and before you posted that, that was exactly what I was thinking.
Thinking of ant tilt as "cow"/arching and post tilt as "cat"/rounding if that helps. Try setting yourself in more of a tucked in "cat".
It sounds like some of your paraspinals were working overtime instead of your deeper Ab muscles kicking in. And with all the "back" exercises you do, it sounds like an Ab "thing". It's still a "core" issue.
I would recommend to someone less experienced to feel the correct posture on knees first, but I'm like you and want to PUSH IT. (but not always; I did knee-down side planks for years before I decided to try full planks and w/in weeks I'm doing full "yoga" expression of leg and arm up as well
)
Hey, look at you! 30 seconds is a GREAT, 100% improvement.
