In my kitchen I have replaced everything with PVC, for the most part, and it's working fine. I am having issues where the J-pipe (PVC) meets metal. I am leaking there despite many attempts at re-seating the pipe and trying again. It is a very difficult connection, and turns hard. It takes a while to get the threads to line up correctly.

What am I missing?

What you
Re attempting to do is marry half of a PVC "J bend" to a chrome tail piece from a metal trap. What should be done is remove the metal tailpiece and convert to PVC.
Good luck Tom