What are some good introductory material on the matter? I'm curious about the different possible definitions and how they might result in differing moral theories.
My views have evolved since I wrote those posts so I don't necessarily endorse everything in them anymore, but hopefully they are useful starting points. For what it's worth, my view now is more akin to the traditional Buddhist view on suffering as described by the teaching on dependent origination.
The section What is pleasure in "Is pleasure all that is good about experience?" by Willem van der Deijl covers a bunch of ways of thinking about pleasure, which you could modify straightforwardly for suffering.