I am trying to cook Buta no Kakuni or Japanese pork belly but it's tough
In his book, Masaharu Morimoto says to cook it for 8 hours for the initial phase (in a 240F oven). He also includes brown rice in the initial cooking, which he says helps to tenderize the meat. He then refrigerates it over night and then cooks it for another 2 hours the next day.  The recipe is reproduced on the Chubby Hubby blog.

If that doesn't get the texture you want, you could try a slow cooker or sous-vide. (Sous vide should be able to nail the texture if you get the temperature/time.)