So when I use shaders such as BSL or Complementary it makes the interior of Tardises lose all of their shadows, is there a way to fix this in the settings of them or is there another pack you suggest (I’ve used sildurs vibrant shaders but I don’t like how it makes the exterior look)

To my knowledge, there is no remedy for this issue. Due to the way the 1.12 renderer works for the tardis dimension is causes these kind of issues inside the dimension. If i am wrong, I am sure someone will correct me in this thread.