This is all excellent advice. I agree that you've probably hit a wall, and it will take some more thought and planning than just pulling away at 500 every other week.
I am a huge fan of GM's and can't praise them enough. The problem I see with your progression on DL day is that you never really train the lift. 135, 225, and 315 as your warmups are not going to up your dead, and 405x1 for a 500lb puller is not really going to do anything for you. Your only real work set, 500, is a miss. Like wnt2bbeast said, train with some 3's and 5's for a while and get your volume and workload in.
Even maybe try 405 for 3 sets of 5, do GM's the next week, do rack pulls to a heavy triple the next, do GM's the following week, then come back and try to hit something big. That is just a suggestion, you have to know your own body, but I honestly think it is more of an issue where you need more workload to train the lift, rather than make every session 'test day', your test day numbers won't go anywhere without any training days.