There are really two questions:
1) How much can you legally carry in a specific jurisdiction?
2) How much can you safely carry from a technical point of view?
If you do not exceed GVWR or either axle ratings, you should always be good to go.
Beyond that, there are no simple, universal answers to either.
If an authority decides to weigh your axles and you exceed any of the three ratings above, you may or may not be in trouble - depending on who weighs you where, and by how much you exceed the numbers.
Technically, the truck can usually carry more - sometimes a lot more.
Only a scale can tell you the actual weights. The payload number is not much more than just a number.
Only you can decide if you think your rig is safe, and which legal or technical risks you are willing to take.