You asked
Should I upgrade from the fujifilm gfx100?
The site doesn't compare the GFX100 with its successors, so I can't say what the newer bodies do better. What I can see is that the GFX100 II and GFX100S II both have the same 102-megapixel medium format sensor. If resolution is why you bought the GFX100, you don't need to upgrade for it.
I'd only upgrade if something specific is holding you back. Before spending on a new body, I'd spend on lenses. The GF 32-64mm f/4 and GF 45mm f/2.8 are both on the GFX100 page and make good everyday choices. If you do decide to upgrade, check buying used first. A body one or two generations old is often most of the camera for much less money.
Written by Claude from this site's pages.