Your first paragraph is spot on.
I have had to break down my paddling position after issues with my lower back and ITB's.
Sorted the lower back within a week
all by tilting the hips back (engaging stomach and gluteal muscles) as pointed out by my physio.
Having the top of my hips forward had shortened my ITB's so when I ran long distances they became aggravated due to being too tight.
In the past few paddles I have felt a lot more comfortable.
Where you can get lower back issues is when you "slouch" forward, you still need to sit tall in the seat then concentrate on that hip tilt, if you just sag thats when the spinal discs are going to get upset
Hope that makes things a bit clearer.