Crispy pork belly, how can I cook the inside more?
It might be sacrilege, but how about microwaving briefly? The crust won't burn anymore, but the heat will help render the fat. Worth a shot. But probably not the best flavor.

I'd probably experiment with pan frying some slices at different temperatures to see if you can get the meat and fat done before you over char the crust. Try slicing it vertically into 1/4" slices with the crust on the top 1/4" edge. That will minimize the crust and maximize the surface area of the inside.