You don't have to. The movies are comedy focused whereas the drama is a little more serious. The characters are different with the exception of a few cameos.
Thank you so much for making me aware that the Gachiban Franchise is available on Amazon! I bought a DVD with couple of the movies a few years ago and just assumed I'd never see the rest.
Poor writing or poor translation? At the end Kento is arguing that revenge is the only reason he is alive, he has no other purpose and nothing left in the world. 37 seconds later he is telling his father that each individual has a value even if their lives are considered mediocre?
My favorite actor. I'm very suprised you didn't include N no Tame ni (https://kisskh.at/11051-n-no-tame-ni) as it is one of his best characters and a great drama.