I understand that the main part of the SRBs are a hollow core of
fuel such that the SRBs burn from the interior towards the casing.
I also understand that this burn pattern produces an increase in thrust
as fuel burns. So as the vehicle becomes lighter, thrust increases.

Why did they chose this pattern? Others are possible, so why is this
pattern used? Finally, does anyone know the variation between thrust
at launch and thrust just before burnout?

-Thanks